I have two dogs, one of them's three-legged (we rescued him that way, he didn't lose his leg on our watch) My three-legged dog is more athletic than my dog with four legs! haha
Weird, one of my best friends in college had a three legged dog. Lost his leg when he got stuck in the birth canal, and the vet pulled on it to get the pup out. To add insult to injury the vet thought he was dead and was about to dispose of him before he came too. Scrappy little bugger!