In your case? The big talk has to happen across the 3 of you.
If the new config is changing to a V with the guy as a hinge rather than the triad originally imagined, y'all need time to process.
Mourn what has ended, and welcome what is new with some sort of grace.
What kind of home are you in? A house? A flat? Do you all maintain separate bedrooms? Moving out to better accomodations doesn't mean you give up on the whole thing. Shoot, maybe they move with you to a floorplan that is more ammenable to a V config!
But before you get to THAT place, you just have to man up and get the talk TALKED. So take a deep breath and work on your stuff together.
Everyone is feeling various emotions. ID them. You can't help what you feel when you feel it.
But before you can CHOOSE how to act in response to them (and we CAN choose how to act and behave even if we don't choose how we feel)... you need to ID what we're responding to here. Make sure everyone speaks their truth, gets heard, and needs/wants are addressed and solutions negotiated.