Still think it is easiest to talk to the trio in trio.
Spill it and be honest about the need to reduce your stress.
Maybe you all agree to play it cool at the dinner. Maybe she offers to bow out. Or he does.
But you have your first opportunity to work as a TRIO on a life problem and you want to do what?
but I hope talking to him about it beforehand, at least expressing this it's difficult at times since this is new to me, will help somewhat. He's generally a sensitive guy.
around at BF to deal with it and take it off your hands? Rather than tell him you made a mistake and tell her you made a mistakes? And own it and ask them for their help to solve it?
Polyships go a lot smoother when people just communicate clear from the beginning. You are not a terrible person for being newbie or making a mistakes. You will outgrow your newbieness and mistakes happen to anyone. But this is the time to set up foundations for how the trio expects to deal with the life stuff that comes up. You can do it. Just make the calls you need to make.
Solve it together and find the happy medium that works for all on this simple dinner prob.
You can do it!