I once hit a deer where both its front legs were paralyzed. The thing was able to scoot on the ground while laying on its side and kicking with its back legs almost as fast as i could run. I would think that a deer on three legs would have no trouble at all covering a lot of ground before bloodloss or whatever else would have it virtually unable to move. In fact some three legged deer occasionally go on to survive and folks run into them again the next season.