I'm sensing some insecurity in your arrangement. That's ok - we all have various levels of insecurities we have to work through. I think that's a big purpose of having 'rules' in the first place - to shield people from their insecurities.
Now it's a little hard to be sure from your post but I'm assuming you 3 had all slept together prior to this incident ? Correct ?
Because I think that matters. I think it's sensible for many 'couples' to want to be present together for at least the FIRST sexual exploration. There s always a lot of insecurity around that.
However, I think that beyond that first time, and if all went well and it was a positive experience for all, a rule like that needs evaluating. It has the potential to take the spontaneity out of the sexual connection, and I think even you would agree that spontaneity can be one of the nicest parts of sex.
So - again assuming this was not the first time - I'd step back and ask myself.......
"Why (truly why) does it matter " ? What am I afraid of that I might be left out of ?
Try for an HONEST answer - then discuss it openly.
The trust broken thing............well, I think there's a lesson in there too. Or two. Maybe about alcohol (or other loosening agents). Maybe it's about clear communication and how different people will understand the same words/terms differently, and consequently potential varied actions resulting from that understanding.
Seriously, I wouldn't turn a molehill into a mountain here. I think there's no real harm done, but an excellent opportunity to see how things can go askew despite the best of intentions. Lessons. Talk it out, get some further clarity and let it go.