There is a reason the saying, “three’s a crowd” exists. Dyadic (meaning two) in nature, people tend to pair up in friendships and love. When there are three people in any long-term group, the dynamic can become unclear and competitive.
As everyone wants to feel a sense of belonging, members in groups of three sometimes go to great lengths to vie for their position. Often, one or more people feel excluded at various times in groups of three.
Three becomes a particularly dangerous number when it concerns marriage or committed partnerships. And when a third entity of any kind (even one considered good) is thrown into a system designed for two primary parties, trouble in paradise will very likely result.