I am not an expert or anything, since my 3 1/2 boy is in the same boat. but I will tell you my mistake and hopefully it will help. I tired everything, and every time he wanted to go I would say, you'll be such a BIG BOY if you go and other words of encouragement. I would go on and on about how it would be great and he would be a BIG BOY. We'll I talked to my daycare person who is studying to be a child psychologist. And for my case was basically doing the wrong thing, it might work for others. He stated that some kids don�t want to let go of the diaper because they still want to be part of a baby. And not grow up. Sure enough when I asked my son, he stated that was the reason. I stopped using BIG BOY or any words of encouragement that deals with growing up. Still to this day he tells me that he doesn�t want to grow up too fast. Well it�s been 6 months after my learning�s, and now and I talked to other parents, and literally today it worked. 1st time # 2 on the potty. 1st we told him that we ran out of diapers and my husband went to the store and they ran out to. Then there was a toy that he really wanted and I stated that if you want to play with it you need to go #2. Last night I was playing with the toy and won�t let him play with it.
Today at daycare he stated that he wanted to go after being offered a prize/toy if he went #2. He really wanted to play with the toy so he finally went #2. Hopefully it will last.
I am not sure if this helps, but good luck.