When my younger son was 11, he broke his right tibia and his left ankle at the same time in an inline rollerblading accident. He had surgery on his ankle and was casted up to his knee. His right leg was casted to his groin. We used a wheelchair for the time he had the long cast. (He graduated to crutches with a walking cast on one leg and a ortho boot on the other. Yeah, he was a sight!) You can rent the wheelchair for the time needed and, generally, your insurance will pick up the tab. During the day, my son showed his independence and would drag himself down the hall and up onto chairs with his arms. Now, remember he was 11; not, three years old -- so, of course, he was bigger and stronger. As for the potty question, I don't have any answers for a three year old. For bathing, we just took large trash bags, wrapped them securely around the casts, taped them with heavy-duty wrapping tape, put a plastic chair in the shower/tub and installed a hand-held shower head with an extension tube. He could then wash himself and rinse off all from the chair without any problems. It takes alot of improvisation for times like this!
Good luck to you and your little guy!
R.