Have you tried talking to him right before bed? And putting him on the toilet last thing?
This is what I would try:
Ok, Timmy, since you have been pooping right after you go to bed lately, we are going to the potty the LAST THING before bed. That means, after story, etc you will sit on the potty one last time before bed. And then, after that, you won't have anything left to go in your pull-up. If you do, since it will be so small, you should be able to clean it up on your own. If the pull-up needs changing after you've to bed, then you will have to change it and clean up on your own.
Then, after the entire bedtime routine, have him go sit on the potty. Stay with him and make sure he really does try to go. Encourage him to try, "because he needs to get it out now, not after he gets in bed, because if he does it after bed he'll have to clean it up on his own."
Then, if he has success, praise praise praise, and tell him that's great. Put him to bed. The end. If he goes in his pull-up follow through. Just tell him you're sorry, when he is done cleaning it up, to wash his hands and get back in bed. Don't go help. (After he eventually goes to sleep, you can go check the bathroom to clean up any mess that was left not properly cleaned).
It won't hurt him to do this. And may solve your problem the first night---when he realizes that you MEAN it. At 3, she should be able to manage his bowel movements enough to hold it overnight. Urination is another "animal" altogether for some kids... but the bm's he shouldn't be having "accidents" at night. You said yourself, he is doing it intentionally.
Good luck.
Then