Three is a very hard age especially I found with boys, endless energy, testing their boundaries and finding out they can try and control things. It is to some degree very normal. I found when my kids were going through growth spurts and not getting good sleep was what set them off more then anything.
Also with boys I learned that putting up visuals is a really good way to keep them on task. Put up a chart, seven days in that week for a month or two. For every bad decision he makes a frowny face. Each frown have a consequence, something that will sink in for him, losing time off bedtime, a favorite toy taken away for 24 hours, losing priviledges going to the park, whatever he thinks is important enough use.
For every day he does well, big praise and thank yous have him even put the smiley up himself. When he is good in class, etc, then a happy face. With 7 happy faces in a row, have a special reward, little toy, new book, ice cream out...something that motivates him.
Kids this age tend to just react without thought, if he is angry he is reacting to that anger as boys are less verbal and tend to be more physical in their reactions.
Just really stress if he gets into trouble at school, he will be in trouble at home as well.
Make sure he is getting good sleep, a huge thing that can cause hyperactivity and outbursts. At three he should be getting at least 13 hours total with or without naps.
Make sure his sugar intake is minimal especially at school.
He is testing his boundaries, defining limits for himself. As long as you are consistent, have consequences for his actions and drive the point home what is not acceptable, it will sink in.
Maybe help give him words to use when he is angry and practice with him instead of hitting. Also asking my kids how they would feel if someone hit them worked in their brains. Then teach him when it is okay to talk to the teacher about someone not playing nice with him (not tattling on everything). Make sure you know what the structure is at school too. Do they do time outs, do they listen if there is a problem with kids, do they teach children to communicate if there is a problem?
Hang in there, my son is the sweetest thing however at 3 that was a bumpy time with him figuring things out. I just stayed on top of it, stayed consistent and HE LOVED the chart system as he could visually track how he was doing. Positive praise worked wonders for him too.