If a male and a female partner go hunting for a single bi babe for them both to "love" and fuck, it generally doesn't work. Usually the unicorn gets fed up with feeling like a 3rd wheel, a dirty secret, a babysitter. She doesn't get invited to activities (extended family or work parties, or any outdoor event where she might be seen or introduced as a lover), she's in the closet, she can't tell her friends she's in a relationship with a couple as they will all think she is crazy. Sometimes the established couple won't let her have sex with them individually, only with them together, or go out on dates one on one, while they get to have sex one on one with each other, or go on dates, whenever they want. Sometimes they have her move across country and into their home, only to treat her like a housemaid, or worse. She may not even get her own room. She may not have other partners. She may not share in finances. She may not get pregnant by the man's sperm.
Now, your situation is slightly different since you and your bf didn't go looking for a hot bi babe to "add" or "share" or "complete you", it just started happening.
So, if your potential gf is allowed other partners, maybe already has a bf or gf of her own, has her own place nearby and her own transportation, her own financial independence, is "allowed" to get pregnant by your shared bf should she want to, with legal financial, practical and emotional support in place, is allowed to be "out" to your family and friends, can have dates and sex individually with each of you, etc., etc., this all could work.
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