He is likely picking up the negative behaviors from playgroup and acting out on them.
What we do (my son is 2, so I’m not expereinced with older kids, but the concept should work) is that when he hits his little brother (9 months and it happened twice) is we do a modified time out.
I put him in the corner, sit down on the floor and we talk. I talk about hitting and how it hurts and how he doesn’t want to get hit. How we have to treat each other nicely. Then I also figure out what triggered it by asking questions. Did the baby take a toy you wanted? If he says yes, I’ll give him suggestions on what to do next time like say “Can I please have the toy?” I also make sure to make him verbalize WHAT he did wrong and to repeat the solution.
About potty training – are you giving him liquids close to bed? Also, wake him up before going to sleep so he uses the bathroom. He will be much less likely to have an accident.