@ lapwing: That's the thing to do. An honest conversation is a must with every involved, except maybe current bf is he's soon to be out of the picture.
Have you slept with either of the couple? I think that would be a requisite before you begin. It seems to me it's premature to count on being a 3rd if you haven't and the other person was ok with it. In my limited experience with polyamory each person has to know what the relationship is with the other person - and keep talking. If the other people don't have experience with it, it's going to take a lot of work to make it work, but the rewards make it worth it. Good luck at it.