I decided to do the whole child led thing and wait until DS asked to use the toilet. He went to preschool September so I thought once he saw the other children he would ask to as well. However DS is 3.8 now (well tomorrow) and the oldest I have known a child start asking is 3.6 and is still not interested. When he got to 3.6 I started to introduce the idea to him myself as he hadn't asked yet. Tried stickers, special treats, chocolate buttons, books about potty training, putting teddy and dolly on potty, saying your friends use the toilet etc to no avail. He's seen me use the toilet and I've even weed in his potty myself
I suppose rather than advice I'm just looking for someone else to say mine hadn't done it by 3.6 either!