The girl you fvcked already, How casual is that relationship? How cool are you with her? If you want to get really ballsy, You can go for the homerun (Threesome). Invite them both out together for drinks with you, Chill out with them for a bit and get some sexual humor convo going, then let them know in the smoothest way possible "I'm attracted to the both of you, Both are very fvcking sexy. Let's go hang out at my place" (Or wherever you're staying) This is if you really don't care but the best that could happen is that they look at each other and say "um... Ok!"
I also notice you're 21, Damn man I'm jealous of you. 21 is Prime time! and whatever you do, ALWAYS use protection (Condoms).