Alexflint is right, in a sense -- the more people involved in a romantic relationship, the more potential points of stress and failure there are. Not to mention, poly people are often operating without a net or a manual, so to speak -- there's little cached wisdom that might help us specifically, and a wide variety of possible configurations into which any poly group of N people might fall.

It has been my observation that there's also more potential (if not in direct symmetry with the increased failure modes) for coping strategies, supporting those in a difficult time and generally things that make a relationship robust. Some drama is harder ("you aren't spending enough time with me and all your other partners are getting your attention"), some is easier ("I have no interest in seeing/doing this with you"). Eliezer mentions the comfort he gets knowing that if he can't do something with his girlfriend, she has other paramours who are happy to do it instead.