Okay, today I had a long talk with him, although unfortunately she wasn't there, and things are not so nearly set in "they walk off into the sunset without me" stone as I had feared. We've sort of come to the realization that none of us are really in the moving-in stage in the relationship and we're only doing this because we're afraid of missing the boat, so while nothing is bindingly agreed it's sounding likely that we'll continue to live in three separate places. Which is probably for the best.
He's also not a sucker who's going to go "oops, I accidentally turned monogamous," without knowing what happened, and she's not an evil manipulator so much as someone who's just nervous about feeling like she's obligated to accomodate anyone her boyfriend dates.
I'll miss my Utopian Poly Commune fantasies, but maybe this isn't the time (and maybe these aren't the people?
) to live those out.
Thanks so much everyone here who's been offering their support.