i also have a 3 yr old son that just finished potty training. going poo is the hardest i think . what helped my son was (and most may disagree) giving him negative responses to accidents in his under roos like "eeew thats yucky dirty only dirty boys do that" i also disciplined him if he went in his underwear intentionally. he got to a point where he refused to poo in his underpants. also when he did use the potty i over exaggerated my enthusiasm like i just won the lottery and he really liked that . but what helped the most was peer teaching my sons in preschool so when his peers did the deed he to wanted to follow along. if your child isn't in a school setting then maybe a brother or sister could help. kinda like monkey see monkey do. hope this helps spark ideas for you good luck
If your son has no bigger siblings, it's not uncommon that at age 3 he would not be potty trained. My pediatrician once said "Only children, especially boys, especially smart boys, just seem to take their time and often are not interested in potty training at 3 or even later." I don't agree with the idea of shaming him as a person with statements about being a dirty boy, but if you want to show your annoyance at having to clean up a mess, I don't think that makes a kid feel bad about himself. But I would suggest instead simply going back to pullups and night diapers, until he gets a bit more ready. In the meantime, look up some of the great resources out there on potty training, or go to the bookstore and get some of the best recommended books.