I totally agree with you , this is not a good thing, the girlfriend should not be hitting a child of 3 years old, speak to your son, I personally do not agree in any way with hitting , she will do it to others at school, most child behavior can be sorted out with giving more attention and focusing on their positive side,The problem is in the child/parent interaction. It is good she has you, children of this age want love and kindness and they respond ,,discipline and hitting them is not the way .