The three-legged hamsters I've seen seemed to get around as well as the others, but they weren't obese. My concern would be the weight (spherical hammies have trouble getting around sometimes even with four legs, and she's missing one!) If they have her on a seed-mix diet, I'd watch her really carefully and make sure she's eating a decent mixture--and if it's one with a lot of sunflower seeds (or just lots of seeds in general), switch her off it entirely (waaaay too high in fat). All-pelleted diets are usually the best (especially in the case of already-tubby hamsters), supplemented with occasional fresh food, but sometimes the hammies don't want 'em.