I had a friend who had a son that wold not poop in the poty either. He would go all day at daycare and then get home and ask for a pull up, hide behind something and go. They battled it for a while, then were advised to just let it go....he was obviously not mentally ready to be 'poop trained'. The battling and frustration was setting him back further and further. They relaxed and let go and within a short while (I don't mean days) he was going in the potty. Every kid is diferent and he will come around in his own time, he just wasn't ready (and still isn't) at the same time that he pee trained. Don't fret, he won't be 16 and still pooping everywhere :-)
Is there any chance of negotiating an alternative 'okay' place to poop? Obviously the potty would be easiest, but since he won't, what about a potty chair, a smaller portable pooty chair, a pull-up, a new potty seat on the big potty??? Maybe you have tried all of these, but I'm just imagining that maybe if he is the one that makes the decision after asking him which one he would feel comfortable doing.....maybe sort of siding with him and explaining that you understand that he doesn't want to go on the big potty just yet, blah, blah, blah, but that you want to ask him where else he would like to go besides the floor, underwear, etc.... At least it might be a start? I agree with getting sick of cleaning up poo from places it shouldn't be, that's why maybe having him pick a place he'd be willing to go? Does he have to help with the clean-up? That may change his mind about pooping everywhere. Either that or revert back to pull-ups f/t until he decides.... I wish you the best! Lots of kids aren't ready by 3, especially boys so try to relax (even though its hard!) and remember that he just isn't emotionally or mentally ready and he isn't actively trying to make your life hard! He's just being a 3 yr old :-)