Dogs can adapt easily to having three legs, especially if it is a rear leg that is missing. Dogs put more than half of their weight on their front legs, so if a front leg is missing they’ll just need more assistance doing things like getting in the car or going up stairs. Getting a harness with a handle on the top can help if you need to ever lift your dog. It comes in handy, too, when the dog becomes geriatric and has arthritis or hip problems. Getting up from laying down will become more difficult.
Outside of that, the best thing to do is treat them like a normal dog! Normal exercise is still important. In fact, obesity can cause major problems for three legged dogs, so don’t be afraid to let them run! It will actually be much easier for them to hop around quickly to keep their balance than walking slowly, so just make sure they take breaks.
I’m sorry you and your pet have had to go through this, but he will be fine with three legs. I’ve met many tripods that are 100% happy and healthy!