Poobah, sorry to hear about your struggles.
In many ways I understand exactly what you are going through. Quads are complicated that way and in my experience (my wife and I are in a quad with another couple), relationships between metamours I find to be extra challenging. I guess I find them to evoke more far more emotion such as anger when encountering challenges.
The push-pull you are feeling is something I have definitely had to deal with many times. If you try to avoid being caught between them you get accused of not being supportive. You walk a fine line with a tons of broken egg shells behind you.
But here's the thing... in the moment it's easy to say "i'm done! I can't do this anymore". But what tends to stop you from making that decision is what you lose. It's only once you calm down you realize it. I heard that statement before, heck I have even said it myself. But no one in our quad can bring themselves to announce it because deep down everyone deeply loves each other. Most of the time a short break is all you need. You have been together for 3 years... despite the ups and downs there is probably a lot of love there.
What I find the best to do is to focus on my wife and allow her the time/space to vent (not an easy task :-)). I try to hold back my opinions and listen. Then as she calms down I can slowly introduce my thoughts to the situation to defuse the issue. And oh ya... I don't speak ill about ANYONE cause that always comes full circle somehow :-).
I don't quite relate to some comments here... when problems escalate and anger and hurt feelings arise, a lot of shitty things get said such as threats, ultimatums, boundaries, etc... I only consider these comments serious when people are calm and have had the time to consider their wants/needs.
Sounds to me the girls may need a weekend away to reconnect.