My hackles are up because I'm having an emotional response to the idea that these three women and your husband were supposedly in a poly constellation and yet none of them had the ethics or pure human consideration to speak to you. What if you do put your foot down? You've been given the ultimatum of him leaving you. You're at home with your six month old child...when does he have the time for three other women as a new and responsible dad? Are you happy with him? Wil you be able to trust him again? Is he worth it?
Sorry to sound so negative...I'm stuggling with the concept of doing this to the woman he loves and mother of his new child. I know what is is like to betray some one's trust...I can't imagine him feeling in a position to ask for anything after this type of dishonesty. And the other women...perhaps he fed them lies as well; otherwise they knowingly engaged in some serious unethical and dishonest behaviour. I don't blame you for wanting them gone.
Hopefully some one without the Don Quixote complex can comment and give better advice. I'll just sit here with the pot on my head stewing for now