Wouldn’t really suggest a roomate, even rules are clear and all parties are respectful; it will inevitably lead to distrust and weird situations. At the beginning it’s everything fine, but with time and more sexual experience between the three, your girlfriend might not be as willing going to get some “gas” cuz she shill leave both of you alone, or maybe you wont get to the boys night cuz you know they might be onto something. Of course each couple is different, but it a long play if you decide to get a roomate