Hi Jake - and welcome.
The situation you describe is far more common than you might expect. I have been in it myself as has my mate. It's pretty delicate. The odds of 4 people matching up are pretty high. It's why various combinations of 3 are far more common.
The ticklish part seems more to be how to be absent from the picture without hurting the other person's feelings. At least that is how it is with us. I wish I had an easy solution for how to approach someone and explain that the sexual chemistry just isn't there for me even as much as I like them as a person, friend etc.
So if things are going along for them as a triad and nobody is pressuring you to get involved, you may want to just say a big "phewwwwww!". At least it gives you time to figure out how to cross the no attraction bridge. Because you can believe that you WILL have to cross this bridge before long.
This is a thing about couples potentially mixing with other couples - or moresomes. The larger the numbers, the more it's likely that someone will connect and someone won't. It's just human nature and one of the tricky parts of living poly.