The age thing bothers me, and I am a woman who has had an ongoing relationship with a woman 22 years my junior for 6 1/2 years.
Your bf is 45. You are 28 and came into the r'ship with 3 young children. Did he take you under his wing, help YOU financially, give you a place to live? Were you just out of a failed r'ship with the father of your kids? It sounds like you have full custody. If that was the scenario, he is repeating it. He has white knight syndrome.
Then your bf (I will call them Jack and Annie, those initials don't work for me), Jack, has the idea to go unicorn hunting. He finds Annie, you 3 have 3way sex, she cheats, she is dumped. Some time later, Annie is back in the picture, repentant and promising not to cheat. But you suspect she is cheating again.
Also, I don't know how rich Jack is, or even if you have shared finances, but I'd resent it if he was spending so much of OUR money on this young woman. Insurance, dr's appts, even considering setting her up in her own place?
Another thing: how does his teen child feel about him having a 21 year old lover? About Annie coming and spending the night in his bedroom along with his 28 year old gf (you)?
One benefit of Jack helping Annie get her own place would be, they could fuck privately and not disturb your sleep or peace by fucking next to you in your bed, since there's no other available room for it with 4 kids in the house. Her having her own place would also reduce temptation for her to fuck her roommate (if she is) somewhat. But then again, she could have other lovers over whenever Jack is not expected.
Personally, I think Annie is far too young to know her own mind or to be able to truly know what she desires. I think it is different from my situation, since I was 54 when I met my gf, and she was 32. She was/is a full grown adult with lots of relationship experience and her own financial support. The situation with Jack and Annie seems more like a power imbalance based on gold-digging on her part and white knighting on his side.
Oh, there is so much going on here that is unsavory. I could go on, but I will stop there.