Serious question here. What or how would you handle your 3 year old son who hit a girl in the stomach and hit another kid over a toy?

My g/f just called me up to tell me that her son had hit a girl in the stomach and also another kid over a toy. She’s really upset about it and I would like to suggest some differant opinions for her. He has been doing some crazy things lately and really the first time he’s been showing this type of behavior. So any advice or suggestions would be great.