I wonder if it would help for them to read what you've written? Or a version of it; history as you see it. Perhaps they can add how they see it and then you can sit and talk about it. Looking at the furture might be obtained in a healthy way by looking at where things changed or how they could of been done differently in the past.
It sounds to me like a case of everything moving way too fast. I don't know the time line, but it sounds fast. I'm not a big fan of moving people in just because they are having problems. Living with people is a huge deal, especially loves. To me it warrents a long drawn out process based on it fitting, not roomate trouble, money problems or anything else.
You could ask that this temporary move be over now and she find her own place, you could move or he could move. Living all three of you seems to be an option that has come to an end.
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