There is a very sweet dog at the shelter that was in an accident and only has 3 legs. Are there any specific behavioral or health issues I should take into account when deciding whether or not to adopt him? Are there any activities he won't be able to participate in?
Most three-legged dogs get along just fine. The key is to treat them like a normal dog. They don't know to be sad about it, so you shouldn't be sad either. They may have some confidence issues you should help them work on, making them feel like the best dog ever! If it does have confidence issues, then other dogs are more likely to pick on it.
The biggest health issue I see is weight. Make sure they stay a good, healthy weight so that they don't put a lot of extra weight on the remaining three legs.