Thank you for the suggestions so far. I have two other concerns/fears though:
1. I made myself a promise that I would not date/hookup with any more guys until AFTER I experienced an MMF or two. This has put me in the awkward position of bringing up MMF immediately when first-messaging guys online. I've only had one or two remotely positive replies so far, though - and the conversation hasn't kept up well even with them. My biggest fear is that finding an MMF is going to take so long, that I could spend a lot of time rejecting attractive male partners simply because they can't do what I want them to do. Hopefully my "promise to self" doesn't turn into a self-imposed deprivation that goes on for a year or longer!
2. The flip-side of trying to find something more elusive in the dating world (like MMF), is that when an eligible partner or couple finally does come along, he/she/they may be a major compromise in terms of appearance, personality, and even sexual chemistry. I know I felt this way with the first woman I slept with; she was far from my ideal choice in terms of body type or demeanor. I felt I was only choosing her because she was the only one who replied "Yes" to my show-me-the-ropes message I sent out to dozens of local girls.
The second lady I met was a better match, but I had to travel to the nearest big city to meet her.
All in all I'm worried that when an MMF threesome finally happens, there are going to be so many "compatibility compromises" involved that it won't be enjoyable. I hope my fears in this regard are not going to be confirmed, because nothing makes me angrier than confirmed fear!