I just did this with my 3 year old who had the same problem. It took a couple of weeks for him to go for the first time, but once he did, that was it, he's done.
I went to Target and bought a bunch of "poopy prizes" Most of what I got was from their dollar bin (summer and pool toys), but I also got two larger toys, one being a toy he has been asking for (a bug vacuum). I showed him the toys and told him that they were "poopy prizes" and that he would get one when he poopied in the potty. Every time he would go in his underwear I would remind him of the prizes and tell him I wished I could give him one. Finally, one day he told me he had to poopy and he went in the potty. Of course, we all celebrated and he got the first big prize. After that I would let him pick from the bag of the smaller prizes every time he poopied in the potty. He was waaay into that. Once the small prizes where gone he got the "grand" prize, the bug vaccuum and we have been in underwear every since (he even asked to go potty at the pool and went). I have been giving him suckers for pooping since then (it just happened recently), just so he doesn't go from prizes to nothing, but we will phase those out too.
Also, a friend told us to put him on the potty backwards to go poopy. They feel a little more secure that way. The first time he went he tried the regular way and wanted to get off. I suggested we try it backwards and it worked great. I think this really helped too.
I know it's frustrating, we've been dealing with this since last fall also. That's why I gave this a try. It took 2 weeks, but worked!! Good Luck!!