Is there something that he really wants (a toy, etc) that you could set as a goal. Once he reaches the goal, you positively reinforce him with that reward. Sometimes this has worked with kids - (one of my children was motivated through this method).
Yet sometimes that doesn't work. For example, my oldest daughter and a good friend's son both had similar situations - once they were 4, they figured that they were too old for diapers & that they needed to go potty just like a "big person". I sympathize with your frustrations over a 3-year-old who won't poo on the pot - but, I believe, everything will eventually work itself out (no pun intended).
Hear about the constipated engineer? He worked it out with a mechanical pencil.