Our inclusion/formation of a MFM triad was heavily negotiated. Even now, the way I think about it differs a lot from the way my partners think of it.
Some facts - the day before anything started, here was the status quo:
F. Me - solo
M. T - solo-ish, in a roommates-with-benefits with E, but not entirely his preference, gender-wise
M. E - solo-ish, in a roommates-with-benefits with T and identifying as bi.
The day after:
Me with T
T not with E
E not with T
Me with T
Me with E
E with T
(There was probably an easier "letter description" of all that, but I don't understand all those just yet.)
How we got from after to now was through two months of turmoil, hurt and E dragging us toward the end of the world. As redpepper mentioned the theory of invitation not demand, those two months revolved solely on E's all or nothing demands and refusal to negotiate in good faith. Once we got him to sit down, put his ideas into words that T and I could read and dissect, we were off and running.
Ultimately after a LOT of talking, T and I decided that rather than lose our best friend and all of the adventures we were planning together for our lives, the three of us sat and talked through our ideas, expectations and desires. For T and I, our rationale was, if we don't invite E in and at least try, we'll never know if this was the solution we needed to the problem of how to go forward together.
And, crazy enough? It's worked so far. We've had bumps in the road, both emotional and practical, but we're trying to talk and work through things rather than allow feelings to get bottled up and ferment.
Of course, relationships grow and evolve and I can already see something on the horizon where it will be another issue of invitation into a different level of intimacy. We'll probably just have to have a sit down talk about it again and go from there.
But, yes, presumptions/demands are bad. Discussions/invitations are good.