Disclaimer: Everything I'm about to say relates to MFM action with heterosexual males.
I'm sure the MFM scenario happens quite a bit in the hobby, but obviously not as much as the 1 on 1.
Personally, I've enjoyed the MFM with Amber and Justin and it was great. Since Amber and Justin are together, the safety issue is pretty much taken care of.
I have read other threads on terb where guys who know each other have arranged a 3some with an SP that they have both had previous encounters with.
I think this is a great idea and could be great fun for all involved but I don't really know anyone else who hobbies, so it would be difficult for me to arrange.
It might be fairly easy for an SP to arrange by simply asking a couple of "regulars" if they're interested. If they trust your judgement, they may be willing to take part without knowing the third party beforehand.