Most three years olds certainly will understand what they are doing is wrong if you tell them and why. But you are right in that they don’t know how to control their feelings and sometimes themselves. But that is true of many adults too.
I am not sure that's the case. Children are very good at putting things into categories. If you give them a list of things that are right and things that are wrong, they will certainly remember it in the same way that they memorize the names of the trains on Thomas, but I'm not sure that the category "right" and "wrong" has any particular meaning to them. It takes quite a while for them to develop what is a rather sophisticated concept that many adults struggle with.
In any case, even if they are aware that something is 'right' or 'wrong', I don't think that doing something that we've defined as wrong for them deserves 'punishment' in the way that you would punish an adult who has a fuller understanding of actions and consequences.
I don't punish. Children can certainly learn what makes other people sad or angry, and have a sense of fairness. I don't think there are any metaphysical aspects of right and wrong that kids have to learn.
