Ethical obligation to pay for kicked out roommate's rent?

In order to not appear to be acting out of greed and thus be more able to smooth over their resentment when you don't pay, you could offer to cover a third of the shortfall if you start getting a say, starting with the immediate veto of the 3 months rent for prospective flatmates.

Agreeing with everyone else, except that to my mind kicking C out was absolutely the right thing to do and I don't see any particular reason why A and B should've consulted you about it. I also don't see why you should be at all concerned about C's room going empty or about A's expectations for what a new roommate will pay because -- after all -- you're not financially on the hook if nobody's paying rent on that room. Just pay your rent and leave it alone. This is not your problem.

You're not obliged to fund the 1/3 share until they find a new roommate. If you are so inclined and see a way to make it work, I think insectosaurus' suggestion that you offer to help out with the utilities is a good one, but don't give it away - you should negotiate something of value in return, be it a veto on the new roommate or they clean the bathroom for a month. This shows good faith on your part. Don't agree to do the rent though, this sets a nasty precedent as AlisonM says and suddenly you're living in a 3 person house and A has a nice new study.

under a control freak like A... sounds more like a passive/aggressive control freak... the worst kind

Agreeing with everyone else, except that to my mind kicking C out was absolutely the right thing to do and I don't see any particular reason why A and B should've consulted you about it. I'll chip in here to say that communication among roommates, and friends, is still very essential. I'm assuming that you are friends with A & B, and this is not just a purely business relationship. Communication that they were thinking of kicking out C would be appropriate because it respects your feelings and inclusiveness in their friendship. By not telling you, they are showing they don't care about your opinion. As was pointed out above, how you react to this will probably affect your relationship / friendship with them. Personally, I would be upset if a roommate made a change to my living space without consulting me first. Even if it's not my personal space, the fact that C was living there meant they had an impact, again assuming more than a purely business relationship. I would open the discussion by recapping what's happened - they made a decision without you, they stated your ass was not on the line financially. From this, you are inferring that this is purely business, in which case you will be sticking to the business agreement - the lease - you have. If they would like help from a friend, they should treat you like a friend, and respect your input and effort. If this doesn't change things, it's definitely time to move on. Be professional about it, give them notice, and don't let them vilify you for leaving them in a bad spot financially. If they want it to be "just business," then make it just that, and nothing personal.

I nth the take on this that your LEGAL obligation is only to pay the rent outlined in your (sub)lease with A, and that given you were denied any say-so or input in any of these discussions about C and C's empty room, you also have no ethical obligation to pay. HOWEVER, and do not take this as a suggestion you pay just as a warning, I would expect that any good relationship you have with A and B (if any) will cool considerably if you do not chip in. Again, not a suggestion you let them strongarm you into paying, but I would prepare yourself for how to deal with that.

Not only is it not unethical for you to refuse to pay if asked, it would be unethical for A and B to ask you to pay a 1/3 of C's rent.

I agree with the clearly-established consensus: you are absolutely not morally or ethically obligated to pay for a portion of the empty room. I would add that in my opinion, A and B have some major balls (and not in a good way). You say at the beginning "I have people telling me that I have an obligation as a member of the household to help out" with the extra rent - by "people," I assume you mean A and B, right? Assuming so, recognize that they are expecting to have their cake and eat it too. you're "a member of the household," but only to the extent that it benefits them. You're an important enough member of the household to be obligated to cough up extra $ to cover the empty room, but not important enough to have any say in who does or doesn't occupy that room and what their rent will be. In my view, the key questions for you to ask yourself are: Exactly how important is it to you that you stay living in the house with A and B?Badly enough that you're willing to put up with having ultimate authority over the division of rent, roommate disposal, new roommate hunting, etc?Badly enough to put up with having no say in decisions that could end up costing you extra money?How important is it to you that you remain friends with A and B?If you want to stay in the house, then obviously it's pretty important -- life is too short to be in an open-ended feud with someone you live with. If you put your foot down about not paying the extra 1/3, you need to be prepared for the chance they'll take it as a declaration of war, a war which your friendships may well not survive. You might then offer to move out, A accuses you of breaking your sub-lease with her, you say OK I'll honor the sub-lease, and there's nothing in the sub-lease about having to pay for part of an empty room. And round and round the argument goes, where it stops nobody knows, etc.Of course, I don't know any of you guys, so maybe it wouldn't go like that at all. Maybe you could just move out with no argument, no one pissed off, everyone stays friends, no problem. I'm just going off the my sense of the situation as you described it. Are A and B like childhood friends of yours whom you've known forever and always been really tight with? Or on the other end of the spectrum, are they just a couple of girls you met 3 months ago when you responded to their Craigslist ad? So, you know. Factors to consider.

I agree with the folks above that since you are treated as a renter rather then a partner, you shouldn't be on the hook for the rent. You should, however, probably offer to split the extra 1/3 of the utilities since that covers a lot of shared resources that you make use of as well.