I wouldn't like those arrangements either. She should have asked before booking it. And, as others have mentioned, you'd probably all be at risk of getting kicked out anyway. Then again, you and the others could also have told her your minimum requirements before you agreed to let her organize it (if you did agree to that), so depending on how all this happened, I wouldn't say you're automatically out of any responsibility at all for it.
At this stage of my life, I would definitely want my own bedroom, and also no one sleeping in the common area because that restricts everyone else. For example some people might want to stay up late or get up early to have breakfast/watch TV etc. And they'd be waking up the couch sleeper. And three adults to a bed would also be too much for me, even if it's king-sized.
Furthermore, if there are more than three people, I'd want at least an extra half bath.
All that said, many people don't have a lot of experience booking things for a group and mistakes are common. To agree to go, accept her making the arrangements (if you did), then booking your own accommodations (and taking another person in the group with you), you've left her and whoever else is staying in the rental in a mess, without giving her a chance to try to correct her mistake. If she's stuck with the reservation she made, one or more people will have to pay for your and the other lady's shares now, which I could see them not appreciating. It also won't be the same kind of group trip, with two people staying elsewhere. Why didn't you at least speak to her before making other plans? Or do you mean you and the other gal intend to pay for both places now?
From her response, "Suit yourselves," it doesn't sound like she's happy with your reaction, either. I'd consider just bowing out completely at this point because after her actions and your and the other woman's reaction, I could see it being a sour trip.