If your 3-year-old refuses to poop in the potty is it a power struggle or is he just acting out?

No, it's not a power struggle at all. Some children (especially boys) seem to hate to lose body parts (excreting a bowel movement) and it's foreign for them. The best thing you can do is not make a big deal of it as his peers in kindergarten will take care of that. Another suggestion is that kids love to flush the toilet, so get some cheap sparkles from a craft store, sit him on the toilet (he's old enough) and tell him if he goes potty you'll let him put some sparkles in the toilet and then flush. He'll be mesmerized! If this doesn't work, ignore him. I haven't met a grown man yet that still poo-poos in his diapers unless he has a fetish! LOL