Thanks for input, Magdlyn!
Just for the sake of clarity, what I have in my hands is a potential for a vee, not a triad. The couple I met online are separate individuals. One of the first advice I ever got on poly was not to get fixated on a particular relationship formation and let things happen naturally. I did expect having to date, get myself really into the poly circles, go over a few heart-aches and then maybe find a nice little triad or a vee situation for myself, with additional lovers and a steady primary.
What I did not expect is to get hit in the face with a couple transitioning (?) and with a metamour who probably hates my guts right now, having to ask her to find it in herself to share, and accepting that since I have made a royal mess of things, she has every right to refuse.
Me: bi female in my twenties