S.E.
I have a 3 year old and she is fully potty trained. She has been since she was 2. I tried the whole M&M thing everytime she went on her own I would give her a few M&Ms, but that stopped working. So, I went and got her the pull ups that allow her to feel the wetness from her peepee, and I also got her a book about how going potty "Big Girl" is cool! She loved the book! I think she REALLY loved being praised after she went all by herself. She went poop in the potty as soon as she was introduced to do it though... she could't stand being in a poop diaper. My advice is get pull ups (it makes them feel like a big kid), you might want a different sitter...I'm not trying to be mean, but you need someone that will help with this, because it needs to be CONSTANT. Then if nothing works... you might want to consider tough love and go with regular panties! I did that when she started getting lazy and would pee in her pull up well after she had learned to use the potty. They don't like the feeling of it and they realize that's not supposed to happen. lol.
Also, NO fluids after 7pm if they go to bed around 8:30pm.
Hope this was helpful!