Not your fault
Probably the worst thing about poly is seeing two people in a relationship (that you're involved in) falling apart. It's easy to blame yourself, particularly if you came into an existing relationship. Even more so if you were only involved with one of them. Because in a monogamous situation, you would have been the replacement. But this is poly. Each relationship is autonomous...they just happen to be interconnected. Be supportive of your partner, but never let yourself take the blame for any of his relationships gone bad.