How do I get my 3 yo to use the toilet to poop?

I had similar issues with my daughter but I had her trained at 15 months so I assumed that's why she would hold her poop in cause she was still too young but like your son peeing was fine. I would up your sons fiber along with his water intake they sell these fiber gummies that I give to my daughter along with lots of water or else you could give him like one of those fiber one bars I do that as well. What I did with my daughter is that when I noticed she was uncomfortable from holding it in and grabbing at her bottom I would sit her on the toilet and would no let her get up until she went and it was a battle at times I would be with her on her little toilet for more then an hour. Also I wouldn't make the diaper available to your son anymore or else he's just waiting out to relieve himself in the diaper. Hope this helps.