Good luck to you. You have mentioned the two main problems getting a gathering for what you want. First, the lack of interest by the women, and second, the couples, actually it is the men, who are threatened by unattached males.
Where I live in Oz, a smallish town, I wait until I am invited by both the man and the woman before I do a threesome that way I know it is mutual. Most people in this town know of me, even if they haven't met me. I guess the grapevine works.