Bills aren’t 50/50
If your utility bill is $300, it shouldn’t be split up so that you and your partner each pay $75, and your roommate pays $150. You and your partner get to enjoy the big, nice common areas just as much as your roommate; why should you each pay half as much as your roommate does? You’re all benefitting from the heat or the AC or the running water; your roommate shouldn’t single handedly be paying twice as much as either of you do. Cut your roommate a break. That utility bill should be split up three ways, $100 each.