I posted once when I didn't have my password (on a different computer) but that was several days ago so I think that post is lost.  I'll try again.  My dog lost her front right leg at 4 months due to an injury.  She passed a year ago a month shy of 15.  I, like you, fretted about losing the spare so early.  So from the get go I was all about minimizing strain and injury but I also tried not to let that interfere too much with her having a happy and fulfilling doggy life.  With everything said and done, looking back, I think I did a good job on finding that balance.  The key to success is making sure they have fun every day with the day including strength building exercise, plenty of breaks and ideally some non-damaging exercise (i.e. swimming!!, you can let them swim non-stop for as long as they can because swimming builds muscle and stamina with zero joint damage). 

My little Nyaya had amazing strength and stamina up until about 7 years of age.  Her favorite thing on the planet was fetch.  Outside of swimming (which wasn't possible through the winter) I didn't let her go as long and hard as she wanted, but I found that letting her play hard for 10 minutes with a 5 minute break worked well as long as I limited it to two 10 minute periods over an hour.  Everyone told me I was being silly, but whenever I let left her in the hands of someone else she would be returned to me broken.  Not because they were mean but because while in their company she would act like playing fetch for 2 hours straight was the best thing ever.  They would only stop when she could no walk to return the ball or freesbie to them.  I would get her back and have 2 weeks of nursing her back to normal with the first few times involving a terrified visit to the vet.

Being three legged will result in more arthritis at a younger age than would normally be incurred of a dog in similar shape.  My dog was missing a front leg, but a dog in the neighborhood has been missing a back leg since it was under a year.  It has had it easier in the sense of more agility and stamina (losing a back leg is a bit easier because more weight is distributed over the front legs, plus the front legs 'steer'), but it like Nyaya has still felt 'older' than it would of for being in the shape it was (like me its owner focused on regular exercise that built muscle and maintained a normal weight).   But because many pet owners do not make a commitment to keeping their dogs in shape the age that arthritis set is was the same as the age for many dogs in the neighborhood (first signs showing at about 8 or 9,  with more significant symptoms starting at over 12 years of age).

From 7 to 11 Nyaya was a solidly active adult dog.  But it was 7 when I started noticing that the distance we covered playing fetch was retreating and that if I didn't carry her to the park down the road she wouldn't have the energy to play once we got there.  She got IMHA at 11.  That diagnosis is as deadly and horrible as cancer and the treatment took a year and a half.  The treatment involved very high dose prednisone and, once it became obvious that would kill her, strong immunosuppressive drugs.   The prednisone wrecked havoc on her muscle mass and gave her Cushing's disease.  Because of this her mobility plummeted.  Odds are your dog won't be so unlucky and if that is the case your investment in regular, moderate, healthy exercise, will allow your dog to get to 13 o 14 before it feels the affects of the arthritis associated with its debilitation.  Muscle is armor, that is a phrase the University of Washington sports medicine orthopedic surgeons told my boyfriend when he asked how he could minimize his knee pain.  Build up muscle and it will compensate for weak joints.

My recommendations are to 

- build Totem's muscle mass and stamina through regular exercise for short periods to avoiding him get overexerted making him prone to injury.  Take him swimming whenever possible.

- if missing a front leg minimize unnecessary joint damage by trying to prevent him from jumping out of car, running down stairs, leaping off of furniture, etc.  The one remaining front leg is taking a pounding every day through walking (if you don't believe me wait until your lying on the floor and your pup hops its front leg onto your chest and you feel like you have been bludgeoned).  I went to great lengths to prevent Nyaya from putting excessive stress on her remaining front leg by carrying her down the stairs (blocking her from running up them during the with a baby gate ) and putting a old chair cushion on the floor below the couch so she could sleep on it and leap off to race to the door, and never allowing her to leap out of a car (jumping up, and going up stairs was fine, and I think that would be true even if a back leg were missing because its a push up, versus fall down).   

- avoid long walks.  Three legged dogs can move better if they have momentum.  I thought it was more relevant for dogs missing a front leg, but I came across the owner of the dog with a missing back leg walking her dog and I asked her what she was doing and she explained that she was teaching her dog to walk on leash so it would know how to be walked that way in case she passed and someone else had to care for her dog.  The owner is in her 80s and had been avoiding walks because they were hard for her dog.  By walks I mean moving at a slow pace across a distance.  Prior to that she (like me) had always taken her dog to the park off leash and let it trot or go at full speed (I once saw her dog chase a squirrel down, zigging and zagging with the squirrel and kill it...dogs missing front legs can't zig or zag...) .  When Nyaya was very young I took her on a long walk with other dogs.  One mile out she was done and I got to carry her 1 mile back.  After that I drove her to the park to conserve her energy for the part she really enjoyed, the playing (not the traveling).  When she got older and I had more time I got a doggie cart and walked her to the park in it and then let her out to play once there.  I would of loved to have attached the cart to a bike, but she was skittish of me pushing the cart a first so I didn't try. Point is try to have your dog use the limited active time you will allow in the day on awesome fun experience and waste it on mundane travel exercise.  

- find new fun when old play ways can't be maintained.  If your lucky enough that your pup lives a long and happy life there is going to come a time when he no longer wants to play fetch, then no longer wants to swim, etc.  When this happens don't just quit doing anything find a new thing that your dog want/can do and take the time through the day to give them the opportunity to do it.  Nyaya's last year was hard and she lost her ability to do much of anything but stand up, walk a few feet, and relieve herslef.  But I found new fun things for her to do.  At that point just sitting and chilling watching the world go by when I pushed her in the cart and dogs walk by in the park made her happy so that was our outing. I bought tons of squeaking toys that she could sit in one place and destroy them (sometimes in less than a minute).  I also discovered foraging baskets.   A puppy would of decimated it in seconds without giving the activity much of a thought, but they wore out my old Nyaya out while making her very very happy.  I would also just whip her into a howling frenzy in the evenings because I could.  The cool things about dogs are they live in the moment.  So it is possible for you to make every moment happy and awesome because they aren't thinking about what happened yesterday or might happen tomorrow.

- proactively manage the pain when it comes.  When your dog gets to 7 to 8 start reviewing the best in class in arthritis pain management for dogs.  Hopefully there will be some new, better options by then.  When Nyaya was 10 we started her on adequan for her arthritis and it was working well.  But IMHA took that way from her as any injected material could cause a reoccurrence.  About the time she started deteriorating her liver counts blew through the roof so NSAID type drugs were not an option.  My vet felt only opioids were safe, and while these worked, they were not at all ideal.  She also hated strangers touching her so massages/chiropractic treatments were not an option.  If she hadn't had these unexpected troubles unrelated to being 3-legged she would of been more comfortable as she aged.  Odds are that Totem won't be so unlucky and that if you keep him strong and in shape he will live a life as comfortable and painfree as an average dog

Nyaya was part blue heeler and part Australian Shepard.  Totem looks like he has heeler in him.  If so you will have to work and work hard at not letting him over do it but you will also have to exercise him frequently to keep him from going psycho and being a bad dog.  I took Nyaya out for four periods a day until she was 3 because if I didn't she would chase her tail in a circle and act like a crazy dog.  It worked because I forced her to take breaks (sometimes carrying her across the asphalt and putting her down on the grass).  In the house I would toss the ball when she brought it to me, but it was for a short distance and I replaced the carpet putting 1 inch pads down making the floor cushy and forgiving on the joints.

Robyn,

Nyaya's injury was also from a dog attack. Her mother and other siblings attacked her when she was 8 weeks old.  I adopted her when she was 10 weeks old.  Kind of funny and sad, but the vet I took her to to check out her 'injured' limb happened to be the one who treated her after the attack from her mother.  I asked can you read me the information about her attack from her chart?  The vet replied back that it reads "attacked by bitch" 🙂  So I guess Nyaya's amputation was also due to a brutal dog attack.  Her left side of her chest wall was also caved in from the other puppies attacking her.  You could feel it was significantly depressed compared to the other ribcage, but that injury never affected her as she aged.

I don't think the attack affected her interaction with other dogs.  She was always open to trying to sniff another dog out, but she could tell quickly when she didn't take to the a dog and would shy back defensively.  But she also found dogs she loved and couldn't wait to see.  It didn't occur to me that her socialization could be impacted by her experience.  I just knew I wanted a dog that wasn't freaked out by other dogs so when she was a puppy I would always ask other dog owners in the park if my dog could say hello.  If Nyaya didn't take to the dog, or the dog didn't take to her and we ended things quick.  But if she found a new play buddy it was great.  I let her/them call the shots.  

I loved dog parks!  But Nyaya hated them.  I guess because she couldn't keep track of or keep up with so many dogs running around.  I think a dog missing a back leg would do better because it can zig and zag. So we didn't go more than a few times.  I realized she hated and decided I shouldn't take her if she hated it (but I did love it!:).  Given COVID gets in pets and there is kennel cough and other risks of dog parks I'm not a big fan.  Find any park with regular walkers some of which Totem will likely enjoy.  

Lastly I would suggest you try hard to balance your fear of Totem being injured with a desire for him to have the best and happiest doggie life.  The woman I spoke of with the three legged dog missing a back leg since it was about 6 months(hit by a car before she adopted it) took her dog to agility training (with a front leg missing Nyaya could never have done that) and training to track rats in cages hidden in barns.  Apparently tracking rats in cages by smell is a thing now for dogs to do and the dogs have a great time and the rats aren't stressed much less harmed.  Her dog is also a hyper active herder breed.  Herder breeds need alot of exercise, but they also bond strongly with their owners and tend not to run off.   Totem is young and healthy.  If you over do it, don't rush him to the vet.  Limit his activity and learn about what not to do the next time.  You didn't mention swimming.  If you can figure out how to let him swim that would be awesome as it is the best strength building exercise that doesn't hurt joints.

Not related to the rest of this post, but possibly relevant is do not use real tennis balls for playing fetch. Nyaya loved fetch and loved chomping on the tennis ball as she ran it back to us.  We had a friend who was a tennis instructor who kept us supplied in old balls.   As I said I would take Nyaya out for 4 walks a day when she was under 3 and 3 a day up to age 9.  She could probably retrieve the ball 10  times in 15 minute period and loved to chomp on it before giving it back to me.  I expect up to age three she chomped on a tennis ball up to a hundred times each day.  At around 7 the vet looked at her teeth and exclaimed how much her teeth had been worn down and asked me if I was playing with tennis balls.  Turns out the tennis balls are made of a fiber glass that acts like sand paper.  When a dog chomps down on them it sands away the surface of their teeth.  Nyaya's incisors were at worn down to about half their original length and I just hadn't noticed.  Nyaya could kill a tennis ball for a dog (deflate it rendering it useless) in 3 or 4 chomps so we went to rubber/soft plastic dogs for dog.  The lesson was don't use real tennis balls.

I look at Totem's picture and can't help but smile. He is a real cutie and he might not get too big.  Nyaya was around 40 pounds, plus or minus 4 lobs all her life.  I was strict about that.  If she got to 42 all treats were cut off. If I didn't notice until 44 or so the amount of food in each meal was decreased.  On the plus side if she got under 40 treats were increased.  In her last year she couldn't walk far enough/fast enough to go to the restroom to so I carried her outside to use the restroom 4 to 6 times a day.  If she would of found a spot just outside of the door she could of done it herself. But she had to walk around a bit to find the 'right' spot.  So I carried her outside to the area she liked put her down and then let her 'roam' to find the perfect spot.  I couldn't of done that with a dog that weighed more.  Given I'm getting older I think I want a smaller dog next time, but I don't want a tiny dog.  I know it is silly as many dog owners don't have to carry their dogs so much, but it is all I have known so I feel strongly about it.  Totem seems like a dog that will have an active, 'big dog' personality but may weigh closer to 30 lbs than 40.  I think that is perfect!  You get a real dog personality in a manageable size.  Enjoy him and let him enjoy the world, but with moderation!