Is there a specific time frame or schedule he seems to go on (my ds will always go after or during a meal, dd will go after playing outside or running)? If so, or if there is a pattern how about taking things away for not going, instead of rewarding for going. Maybe he doesn't get to play until he goes, especially if you know there is a certain time of day he will go. Twice dd pooped while at the playground, the new rule in the house was that we didn't get to go to the playground until she pooped. Ds had accidents watching tv, so no tv unless he went potty first. And if he didn't go, he didn't watch tv.
If you see him getting ready to go in his pants, and once he gets to the potty doesn't have to go, don't let him play again until he does finally go on the potty. At 3 they can understand he doesn't get toys until he poops. "son i know you have to poop, we are not going back to toys or playing until you finish pooping." And a few tantrums because he doesn't get to go right back to play sure beats cleaning up poop umpteen times.
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