Thanks for all the advice. One of my friends actually did apologize to me before I could say anything to him, so that was nice. I did end up talking to my other two friends and explaining how I was hurt. One of them did apologize and admitted that he was wrong not to stand up for me. I don't know if he would have said anything on his own, but he seemed pretty sincere, and I remember he did at least look sorry when they left me at the end of the night. I'm pretty good friends with these two, so I figure I'll forgive them, since I have never had any other problems with them. Plus, they will still be there when we all go back to college.
I am still a little upset at my third friend. She did say she was sorry, but then basically dismissed the whole issue since everything worked out in the end. I hate that excuse, just because someone gets out of a bad situation through blind luck/the grace of God/being awesome doesn't excuse the person who put them in that position in the first place. If I tried to mug someone, but it turned out they had a black belt in karate and beat me up, I would still expect them to be mad at me, even though everything worked out okay for them.
I figure I'll just let it go for the three of them, I am pretty good friends with them, and it doesn't seem right to end things over this, especially since I would probably still see them when I go back to college. Plus, what they are guilty of is basically being cowards, and I've done plenty of cowardly things before. However, I won't hesitate to remind them of this if we ever travel again. I don't care about the sisters, or what they think. I did know that the younger sister had a very forceful personality, but I never had a problem with that before since it never caused me any problems. Now I know better.