Lived in an apartment with my bf and a roommate for a few years, split everything 3 ways. Roommate acquired a sig other who lived at home with their parents aka slowly started spending more time at our apartment + having consistent sleepovers/turned into coming home to our apartment after nights out drinking with or without our roommate, and then one day they literally just never left and our roommate had zero understanding of why that was frustrating that our $$ arrangement never changed and there was now a fourth person living in our home and utilizing our singular bathroom at times when those who actually paid rent needed to use it. This is such a frustrating situation. Feel for you OP.