How about I stay in an open relationship with my best friend and my roommate?
Why would you want an open relationship with your roommate?
The only reason is that I'm not attracted to her.
If things go bad with your roommate you are going to run into a lot of trouble. Its probably best not to mix relationships with the person you are living with.
It's only a fling and buddy stands no chance.
I can see how this is a convenient thing for you, but it isn't wise to start a relationship with the person you live with. Feelings get in the way a lot of the time even though they say they don't.