My Okcupid needs updating bad, lol. It has a bit of poly community on it, so it's not too unusual there.
Usually the way it works is both my profile and my bf's profile will mention we're in a relationship together, show an "available" tag instead of a "single" tag, and link to each other's profiles. Currently mine says this, but my bf's is blank atm.
Check out orangeyaglad's profile on there for a better idea.
Like a lot of MFs looking for a threesome, I'm sure you'll find some MMs looking for the same. We're looking for someone for a long term relationship.
This will be the first MMF I've tried. I've been in a two couples (MF+MF) situation, a crazy network situation M(1)F(1) + M(1)F(2) + M(1)M(2) +F(2)M(3), and recently a MMM.