I have no experience with 3 legged dogs, but I'd like to add my 2 cents...

We were interested in adopting a 6 yr old Dalmatian from the Humane Society back in 97. We initially missed out on adopting her, due to my dad's reluctantness, and she was adopted by another family. They returned her a week later, saying that the vet they took her to said she NEEDED to have her back right leg amputated because she was lame in it. She had been hit by a car as a younger dog, the leg was never set right, and thus never healed properly. She would use it occasionally, but usually held it up. They were appalled that the HS would put a dog up for adoption in that condition.

Luckily, the HS called us and we quickly adopted her. Our vet (family friend) said there was NO reason to amputate the leg because she was in absolutely no pain. We had her (Pepper) for another 8 years.. all of which she lived on 4 legs. She did eventually develop arthritis and needed Rimadyl, but other than that, she was perfectly content with her damaged leg.

I would suggest a second opinion by a different vet...