Hi! We are just starting this process with our daughter who is 3 yrs, 4 mos. We lucked out with our older son -- he has autism and sensory issues, but he basically potty-trained himself by the time he was 4 (we did the book/video/potty doll routine, but didn't push him). My daughter has more challenges in terms of potty-learning than he did, though, so I know she may take longer. One thing that I know will help her is to make it part of her routine (picture schedules are a way of life here). At her preschool and here at home, we have potty time built into her routine. We go to the potty, but never force her to sit on it or anything else. Right now, we are just trying to get her into the routine of going to the place where the potty is located. We are hoping she will choose to sit on the potty by Christmas, but that is a "hope" rather than a "goal." If she hasn't made any headway by the time she turns 4, then I may look into some of the potty advice I've seen for kids with autism (she doesn't have it, but she "hovers" around the edges of the spectrum).
Don't know if that helps at all, but I thought I'd offer it. Good luck,