For him to be having accidents because he's having fun and doesn't want to stop playing when he'd been trained 3 years already seems unlikely, so I agree, his "accidents" more than likely aren't :-/ You have to outsmart him, mom, call his bluff.
Is there a pattern when he's having these, say the same times of day? If so I would have him stop all activities about 15-20 minutes prior and have him go sit and poop.
I saw a "potty watch' on Amazon the other day for $10, it resets itself again and again, (make it more often than every 2 hours,) uses music and lights and alerts him to go to potty and poop. And if he thinks he's missing out on playing by stopping and going now, the having to go many more times should help him make his choice, go once on his own or many times when he doesn't want to.
http://www.amazon.com/Potty-Time-Watch-Blue/dp/B001AH8JNC
i would get rid of the pull-ups, he knows they hold the poop in, and have him wear regular undies. I don't believe he'd like cleaning those, then cleaning himself in the shower or tub, and having to put the dirtied undies in the laundry, because it will make him miss soooooooo much time playing. And if you have a washer and dryer hide his clean underwear, tell him he has to wait by the washer while you do his dirty laundry and, no, he can't play with no bottoms on, he has to wait ; ) Not punishments, but natural and logical consequences to his behavior, or lack thereof.
You really don't want to have the kid who pooped in his pants at school. The other kids will know, will remind him of it, and he won't like being reminded of it by them. My neighbor's son did it the first two weeks of school and mom and dad were told he needed to see a therapist. He was considered too immature and told to come back the 2nd half of the school year IF he was trained.
Your son isn't a baby, I know you know that, I can feel your frustration. Sit him down and explain that 1) he can't do this at school so he can't do it at home, 2) he'll be earning back his Wii and games and other toys and privileges one by one, say X amount of days with no "accidents" he gets the Wii, another X amount of days he gets a game and so on, and 3) the responsibility is on him.
Again, not punishment but consequences for his actions. Hold off on rewards until he's gone a week or so not having "accidents" and do something really special ~ and hang in there!!