I hope his preschool realizes that it is perfectly normal for some children not to be completely potty trained at 3 years old. The ratio of children to staff does often change when you move up to the three year old room.
However, since he has already shown you that he
can do it, that is half the battle.
Keep praising him when he does so on his own. Keep any accidents matter of fact, but if you think they are partly due to him being lazy, be sure he assists in changing his clothes and cleaning up any mess (as much as is practical). Offer frequent reminders....sometimes they get too busy, or don't want to miss out on something, and wait too long...
