I see she has the full right to bring her child the way she wants. you are not his dad and she did nt ask you to carry his responsibility with her neither financially nor morally. I cant believe you do not love a three year old child ... it is normal for 2-3 years old children to explore the world around them. Doctors call this age terrible twos and it doesnt stop until he is 4 . So bear with it sometime and let her know your point of view. Do you imagine she will throw her kid for you? No , you will never respect her if she did that and she cant restrain a couple of years old kid for you. I am sure if he ws your son, you would have give him all the excused in the world. Search within yourself, do you really like the idea of her having a kid from someone else?