A triangle is more stable than a dyad, but it creates an “odd one out,” which is a very difficult position to tolerate. Anxiety generated by anticipating or being the odd one out is a potent force in triangles.
In calm periods, two people in the triangle are comfortably close “insiders,” and the third person is an uncomfortable “outsider.” The insiders exclude the outsider and the outsider works to get closer to one of them.
If tension develops between the insiders, the most uncomfortable one will move closer to the outsider. One of the original insiders now becomes the new outsider and the original outsider is now an insider. The new outsider will make predictable moves to restore closeness with one of the insiders.
At moderate levels of tension, triangles usually have one side in conflict and two sdes in harmony. The conflict is not inherent in the relationship in which it exists, but reflects the overall functioning of the triangle.
At a high level of tension, the outside position becomes the most desirable. One insider opts for the outside position by getting the current outsider fighting with the other insider. If this is successful, s/he gains the more comfortable position of watching the other two people fight. When the tension and conflict subside, s/he will try to regain an inside position.
If the tension is too high for one triangle to contain, it spreads to a series of “interlocking” triangles. Spreading the tension can stabilize a system, but nothing gets resolved.
People’s actions in a triangle reflect their efforts to ensure their emotional attachments to important others, their reactions to too much intensity in the attachments, and their taking sides in the conflicts of others.
Adapted from http://www.thebowencenter.org/theory/eight-concepts/triangles/