If your 3 year old boy is potty trained and has been using the toilet properly and then suddenly the 3 year old starts peeing his pants then it could be caused by regression.
Your 3 year old may be stressed out by some situation and then is regressing to peeing his pants like he used to do in his diapers.
The 3 year old has been peeing in his pants or at least a diaper since he was born and then suddenly he has to go to the toilet to pee instead of wearing a diaper and peeing in it.
Regression in 3 year olds who are potty trained can occur because of a new baby in the house, moving to a new house, starting daycare etc.
Or your 3 year old may just be too busy playing and not wanting to get up to go to the toilet.
Don't scold him for peeing his pants but remind him that he needs to use the toilet and if not he has to wear diapers.
I have a 3 year old that was potty trained and he suddenly began peeing his pants and did so for a few months so I put him back in diapers and then he just used the diapers again because I was tired of the messes and changing diapers is easier than cleaning up pee on furniture and in the house etc.
The diapers only cost me $30.00 per month and my time is worth more than that.
They are only little for so long so if he needs to wear diapers then put him back in diapers and then eventually he will want to go to the toilet and the peeing his pants will stop.