Sorry to hear about this latest SNAFU, it sounds like G and H are making a rash decision. If you're already expecting, then you could end up with two infants in the house. That's two very-high-maintainance human beings.
If it's not too late, if H is not pregnant yet, then now would be the time for them to switch back to safe sex and do so quickly. I don't know if talking with them about that will help, but I hope it will.
I don't keep track of relationship stats and configurations, and my experience (outside my own MFM V) is limited to forum experience, so I'm not qualified to say what the failure rate is for FMF triads. Off-hand, I can think of two that seem to be going pretty well, one that changed into a "V," and three that went "tits up."
That is, the one female partner broke up with the original couple, but the original couple stayed together. I actually think FMF triads are pretty popular in terms of how many people try to attain them, it's why we have terms like "unicorn."
I think the best thing is to concentrate on your own situation, rather than judging how it's gonna go based on how it's went with other people. Certainly you have a crisis on your hands. We don't know how that's going to turn out. More communication is needed. Agreements need to be stuck to (doesn't mean they can't be re-negotiated from time to time).
I hope your situation improves.