why is a potty trained 3 year old still going on it self both **** and piss?

Answer 1

It could be that he's not fully potty trained or he's nervous

Answer 2

Could it be possible that the child is experiencing some level of stress or anxiety? have there been any big new changes which have occured in the household? Either of these may cause a potty trained child to have relapses. Also have you spoke to their pediatrician and make sure there are no underlying medical conditions? Hope this helps and good luck.