It is so refreshing to read the part where you met someone and started your current triad, because you referred to her as a woman. Halleluiah!
I get so tired of reading posts, "We're looking for a girl to add to our relationship." So dismissive for a couple to call a woman with whom they want a sexual and emotional relationship just a "girl" or "female." It just emphasizes that sense that people are looking for a toy for their own amusement without acknowledging the emotions, wants, and needs of a flesh-and-blood human being -- it makes my stomach turn whenever I see that attitude. I would hazard a guess that this woman is so exceptional that being involved with her has allowed you to mature in your viewpoint about this and see her and yourself as woman, and your boyfriend as a man. Three mature individuals. Congratulations! There will be challenges, but if you both develop separate relationships with her and not see it as two-plus-one, it will be very rewarding. Stay aware -- I wish for you that all three of you grow together in love and support of each other.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein