Someone help me find this one quick. JIm/Simon established relationship, Blair seduced in to join the three of them.
I remember that Jim is hurt (think this might be post TSbyBS), Blair and Simon are both scared for him. Jim and Simon decided they want Blair in the group. There is a part where Simon or Jim calls Blair "Love" or 'Lover" and it triggers something in Blair, freaks him out. I remember they call him 'Professor or "Teach" sometimes.
NM, I *FOUND* it.
Of Sleep and Dreams.
http://www.852prospect.org/archive/archive/romance/ofsleep.html And I was mistaken, it was Jim and Simon having once had a previous relationship that Simon rekindles when he finds himself attracted to Jim's current lover, Blair.