I've used "girlfriend" in the past, though it seems so inadequate. I considered "mistress" and discarded that due to the negative connotations. The "primary/secondary" sort of accounting, while it may be accurate in some degree, feels too sterile to be of much use.
Some friends of ours had a MFF triad. Two of them married and they referred to the third as their "fiance." While I appreciate the thinking behind that, it just doesn't work for me.
Just calling them all "lovers" doesn't resonate with me, either, so I'd love to find a better term. I suspect we're going to have to plunder other languages and coin yet more words to describe our entanglements best.