Ah, the "joys" of poo in the pants. Our 3-year-old is potty trained pretty much. Peeing on the toilette was a piece of cake but pooing has not been. We think that the problem lies in the fact that pooing takes more patience and control than peeing does. Sometimes we have to go poo but we need to spend some time sitting there to accomplish the task. Three-year-olds aren't known for their inclination to sit patiently and wait for anything. So, our son would sit on the toilette, give it 10 seconds and declare, "Done!" Then a few minutes later he would be playing in pooed pants. It's a timing thing. Also, a motivation thing.
So, the two things we've found effective are:
1. Making him help us with the stinky, dirty job of cleaning up his soiled undies--which he really doesn't like to do
2. Upping the ante on rewards. My husband declared one day, "I am so tired of him pooing in his pants [usually my husband is the one who will clean him up when he does] that from now on whenever he poos in the toilette, I will give him ice cream [our son loves ice cream]." These
mini ice cream cones are suitable for this type of reward, I think.
Some people may have huge issues and think, "Well, then your kid is just going to eat ice cream all day!" I say, initially, "If it works, it works" and in my experience, eventually the child doesn't even ask for the reward any more because the habit is already there--they don't just start pooing in their pants again--usually they don't revert. So, give it a try and after 3-4 weeks you may not even have to give the reward anymore.
For going pee in the toilette we had started off by rewarding him with juice gummy snacks--2 for each time he went to the toilette successfully. At this point, he doesn't even ask for the snacks because he just goes to the toilette automatically.
For going poo we had been rewarding him with five gummy snacks and some coins to put in his piggy bank but it just wasn't the right type of reward for the accomplishment apparently. Since introducing the ice cream reward we haven't had one incident of poo in the pants and my son has taken the initiative to tell us when he has to go poo. He is much better able to focus and actually know when he really has to go. It has been a complete success. We just needed the right reward.