itch.io

I did mean a Master/Sub relationship, yes. Though I think you can either be sub for both of them or also be the master of the Roommate depending on your choices!

Without spoiling too much (and only based on my vague recollection, bc I honestly don't like these characters much so I only did this once a long time ago): 
Keep completing the challenges the roommate's partner gives you. Eventually you may get an event where there's a sock on your dorm room's handle. Go in anyway and interrupt your roommate.
A week or two after this event the partner will come into the dorm room uninvited and ask you something. Be truthful with them. This will prompt another scene where you can choose how things play out (basically whether you wanna be dominant or submissive with these two). 
A week or two after THAT event and when entering your dorm room you may get the chance to make your little throuple official, either being a sub for both of them or dom over your roommate along with their partner.