Sameold

I have a three year old son, along with his three big brothers.
He doesn't have empathy until after the fact. It doesn't occur to him that his action will hurt someone until after the person reacts in pain. He's very sorry after he hurts someone, but he can't anticipate what will cause someone pain.
Age helps--unfortunately that doesn't make much difference with your situation where you need the problem solved immediately. Every one of our boys went through this phase of wanting to play roughly without comprehension that they could hurt others. And it was play. The older boys have learned to pull their punches and play-fight, and do it ALL the time. Sometimes someone gets hurt, but it's always an accident in play. (I can tell, if one deliberately hurts his brother, say he is mad about something, I have to demand the apology and it's grudging. If it's an accident, it's 'Oh, no! I'm so sorry! Are you okay? Mo-oom! I hurt him!')