Something to consider...
Something to consider when in poly relationships is that any conflict/drama that a new person brings in doesn't only affect your bf-- it also affects you. Something he most definitely needs to take into serious consideration-- that if this girl does go whacky, he's not the only one paying the price.
Secondly... expecting people that have behaved dysfunctionally to suddenly not behave dysfunctionally is a bit naive. Her behavior was off the wall and it involved the production of children... two strikes right there. To expect that she has suddenly changed now into a good and ethical person is dangerous. Not that people can't change, but it usually takes a big effort, some introspection and time to prove that the changes one has made are real and deep and won't be thrown aside the first time that person doesn't get what they want.
And thirdly... she has three children she's keeping from their father and she wants MORE someday?!
Run away fast.