Agh, I feel your pain. This probably won't help one bit, but I'm in your shoes. My three year old son wouldn't even sit on the potty. Now he's consistently peeing in the potty and we're still working on the other one.
Well, he loved his diapers and wouldn't even sit on the potty to try it out. Not even candy would sway him. I took him to pick out underwear and got his favourite theme characters. I told him he could wear them when he didn't need diapers. Of course, I've been talking and talking about how proud I'll be when he can use the potty, etc. Then I told him the diapers were all gone and committed myself to that.
Then I gave him to my mother-in-law. I was told she gave him a lot to drink and took him to the toilet every hour to try to pee. I don't know how she got him to sit on it. She said to let him pee on the floor, wet his pants, and point it out to him and tell him to use the potty next time. When he didn't buckle down, she had him sit on the potty until he did pee, which took about two hours. Everyone in the house praised him like crazy every time he used the toilet. She said to just keep on asking if he needed to pee and to remind him to tell someone if he did.
He's being more difficult about the pooping. I've been patient, understanding, very praise-full and proud of his peeing accomplishments, etc. Yet he kept pooping in his underwear. I kept reminding him to use the potty and to tell me if he needed to go, and he wouldn't. So one day he pooped and I refused to help him change his underwear for a little while. I carefully explained that he needs to do his poop in the potty and not in his pants and he needs to tell someone when he needs to go poop. He didn't do that and he had to live with the consequences. When he asked to be changed for the third time and looked really uncomfortable I relented and helped him. He started pooping in the potty at his grandma's house. He still hasn't done it for me yet.
My son hasn't been advanced to the next level in his preschool because he's not potty trained yet.
Don't know if ANY of this helps, but I wish you all the best!