What a great insight into what it's like to be a third...thanks for that!
I think about my relationship with my two men and remember how important it was at the beginning that I make sure I keep Mono (the third) up to date on everything that came up for me and my husband. My husband did the same. There were times I was completely over whelmed by it all but I realized that I had a responsibility to both of them and that it wasn't an option to hide in bed. I was (and am) completely open about how I felt as it came up. Sometimes I had know idea what it was coming up, just that I was feeling strongly. I don't know if I would be satisfied with "following the lead." I think I would be a bit more demanding.
I would be asking what the trigger is and working on finding out WITH them. I would want to know what she wants me to do if she deeks and avoids in bed. I'd ask if she wants you to come and console her, chat with her, sit in silence, get the hell out of the house, what?! If you are unclear about what you should do or what your role is, shouldn't you be asking, discussing, setting boundaries. Perhaps it is not okay for you that she does that rather than talk it out? And the should be fine. Radical honesty baby. If you are really becoming a part of their already established relationship then shouldn't these things be out in the open?
What does the husband want? There is no mention of what he thinks and what their already established dynamic is around her repeating this behaviour. It sounds as if she may be accustom to trotting off to bed when the going gets tough.
If you are going to be three then shouldn't you BE IN IT? It just seems that you have put yourself in a position of being the third wheel, rather than the third.