Troubles with roommates

Where to start... I am in college and live with three other students. We started living together freshman year, and have gotten pretty close. I guess I should mention that I'm a serious introvert, and my roommates have become my closest friends. I would be really lonely if I didn't live with them. However, there have always been problems. One roommate has a very Type A personality; she can be bossy and immature, but is generally the one to maintain order in the house. Another roommate is also a bit uptight in her own way, but her biggest issue is being too loud with music and friends, and just generally being inconsiderate sometimes. The third roommate lives downstairs and is very easy going and quiet, and we don't see as much of her, so of course I often end up caught between roommates 1 and 2, who often butt heads. As we're approaching Junior year, it's time to think about renewing our lease. Roommate 2 recently let me know that she doesn't want to live with roommate 1 anymore, and has several friends who would love to take her place. The third roommate is in agreement, because she sometimes feels bullied by roommate 1. They told me this, and I've been mulling it over for a while now. It isn't fair for the two of them to be stuck in a living situation that makes them uncomfortable or feel bullied, so I understand why they want roommate 1 to leave. However, I also don't feel right about ganging up and kicking roommate 1 out of the house. This is further complicated by two factors: roommate 1 desperately wants to get a dog but no one else in the house wants one, and she is also planning to study abroad next year leaving us without a fourth roommate. I guess logically it makes the most sense for her to be the one to leave because of these factors...but it still feels wrong to me. She's my friend. They're all my friends. And I love this house and don't want to leave. What do I do?

I'm so involved at this point, with all the behind-the-back talking, that I don't think I can do nothing. Certainly part of the problem is that we haven't all spoken about this together, but are speaking to each other individually. I guess my moral dilemma is that this feels shady, like scheming. And I don't want to alienate anyone or lose them as friends.