Nothing is ever inevitable. It will undoubtably take a lot of talking and negotiating, but there's no reason everything else can't be salvaged. It sounds like you're ending on relatively good terms.
If AA moves out, that's her choice. Three people in a one bedroom, it's no wonder there are issues. Her moving out, alone, could be something that helps your relationship. You say she's still there because of financial reasons, which leads me to wonder if it was financial reasons that got her living there in the first place. That rarely works out well as a permanent solution.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."