Gosh. Well, she shouldn't have ever hit your kid. I don't care if he pulled a knife on her. BUT, he also shouldn't have hit her. If a 3 year old "intentionally" hit me in the face, and I was the teacher, I would ask that he be removed from the school. Why did you child think he had the right to punch someone? Is he aggressive? If he ISN'T, then maybe he was protecting himself from something the teacher was doing. If he IS, then you need to reevaluate your parenting, since a teacher shouldn't have to worry about a kid hitting her.
I am not judging you or your kid. I don't know you OR your kid. I am just giving you feedback and plans of action based on scenarios. I hope I helped....