go for the 3way with your roomie and her bf. you'll love it!
If you feel violated, or that the guy could do you harm, then yes, tell her.
I have to ask - - just how were you masturbating?
Just open your legs wide and enjoy it. Knowing that someone is watching you is going to make your pussy explode.
if you were sqwitched on knowing they were watching you. then act on it.find out if you can share the bf. then fuck the hell out of both. life is very short. don't let any session of pleasure you might want to try go to waste. if your room mate isn't interested and you still want to try it , go find a couple friends that you can really trust to keep it just between the three of you. then , go down on both and see what you like. when you are sure what you like the best about doing a threesome. i am hoping you are female, and of course will like playing with another female. let me know how you make out. either way you will find out what you seriously are into, and know what makes you happy