An update, for those interested: the three of us have been dating regularly since that post. Actually, we've hardly been able to spend time apart. Our strong friendship has rapidly become a strong triad, and I pretty much adore everything about my girlfriend and boyfriend. Yes, I've still got the NRE going, but I feel pretty confident in our relationship. They've asked me to move in with them, and I'm pretty strongly considering it. I spend enough time over at their house as it is that it wouldn't be much of a stretch. Anyway, just wanted to give an update.
Actually, my girlfriend did comment one time a few weeks ago that she finds it interesting that we so smoothly shifted into being a triad, since most of the poly relationships she knows about start off with just a primary relationship with some secondaries on the side that are not usually as mutually attached to both of the primary partners. Is it weird that a triad feels so natural to us, or that we slipped into it so easily?