Ok...after more than a year of being a MFM triad, we're slowly easing into shared, threeway sexual activity and managing the four relationships it affects.
Am I airtight on a regular basis? Hell no and all of this has come with trust, love and the recognition that there will still be times when we will effectively "pair off" into our duo relationships with each other. We've also recognized that we can handle taking turns being either at the focus of attention or maybe on the sidelines during this shared activity.
If you're hellbent on getting over your threeway phobia, here are some things that worked for me during our limited interactions:
Don't set a place, date, time and second for your blessed first non-psychotic threeway to occur. Let it happen organically. We shared a bed even before we got romantic, so that wasn't a huge deal. After becoming a triad, we occasionally shared snuggle nights in bed. Gradually, over months, more and more clothes came off. Then, one night, with a little bit of discussion as we were laying around naked, we decided that it would be rude for one of the guys to get a handjob without the other. And before festivities even started, there was some quick, independent consultation to make sure everyone was OK with the situation.
Pay attention. Bodies are funny things, sometimes they react before the brain can process things. If someone's libido is flatlining, stop for a second, try to isolate whatever activity is frightening/disgusting/enraging one of your partners and then reconfigure. I'd set your expectations low for the first few encounters and work up to the big bang, if you will.
Debrief after you've debriefed. Talk with each partner independently about how things went and whether they enjoyed themselves/were comfortable/were homicidal - you get the idea. Make sure to listen to what works and doesn't for your partners. A post-game talk shouldn't have to turn into a marathon episode of "The View," but you should be able to gain some valuable insight into your relationship.
Good luck. Let us know how it goes.