I think there were mistakes on all sides. And most of it boils down to bad communication.
You need to talk to your husband. He should be your best friend and you and he should be a team. Any decisions should be discussed together before involving anyone else. He needs to learn to stand up to his parents. They have no right to tell him to divorce you and he needs to make it clear to them that he will not tolerate that. And he should have put his foot down about them staying. If I just had a new baby I would want to be alone with my husband and my child in my own home most of the time. Especially an apartment that is only big enough for three people. They should have understood that. If it was only for 3 days-Id say fine but 3 months? no way!
I am guessing you are both young and both of you have dealt with the situation with his parents badly. You need to support each other and tell your husband that you do not expect him to choose between his mother and you. You understand that she is an important part of his life and respect that but he has his own family now and he needs to put you and your child first.
I recommend some relationship counselling. Your husbands family are trying to turn him against you and you have a baby to think about. You need him more than they do and he does not need the stress that they are placing on him.