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There's a very good book about triangulation and the emotional politics of that by a woman named Harriet Lerner. It's called the Dance of Anger. (She has a bunch of other books called The Dance of Something and I think they all deal with triangulation, but I think Dance of Anger is her first book).
Anyway, it's good to learn about and especially about how having a third leg in a relationship can prevent you from confronting issues in your primary relationship, sort of having your cake and eating it too.