I'm sorry that something seemed to start out so well, turned into a big old mess.
It sounds like your gf is under a lot of pressure. She is quite young, a college student, early 20s at the most? She's got her parents to deal with, lots of schoowork, and she's still trying to figure out who she is, and what she wants in relationships.
Then along comes a married couple with young children. NRE led you 3 to spending way too much time together too soon, given her other pressures in life.
Something casual became serious quickly, and now she is backing off. Your husband is in the throes of NRE and feels he loves this new young chick as much as he loves the wife he has been loving, living with, and raising kids with for years... NRE is so exciting and can lead us to act like insane people if not kept in check. He may well have come on too strong to her (calling too much and giving off a stalker vibe).
Look carefully at the mistakes you all made (especially her drowning in your family life when she really isn't in the place for it). Take your time, breathe, reconnect as a couple, be with your children and give gf some space and time to miss you all. She might come around again. Or she might not.
Triads are super tricky and hardly ever work out longterm (post NRE and superlust times). You're not alone.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37