When people share 4 walls, it’s a good idea to check with others about inviting guests, and to make it clear whether it’s an everyone-together event, or a pair-bonding event. It’s reasonable to want to want clarity about how your shared space is being used & whether you’re part of things. It doesn’t need to be a confrontation; just ask politely if they can let you know, so you can plan to be there if you’re to be a part of things or plan to trek to the library if not. Forced triples are awkward & new for everyone; try to let go of your annoyance, but don’t abandon your own needs in the process.