When a third person joins a pair, the flow of communication, influence, and closeness between the original pair will typically change. People often feel invaded, forgotten, angry, hurt, and resentful when there is no understanding of how to address a triangle and the changes occurring due to the third party's participation. We can consider four types of triangles.

The “I have a secret” triangle occurs when person A passes a piece of information to person B, attained in a conversation with person C. The information is not typically innocuous, tending to hold some emotional weight. The information could be benign, such as “C told me that he respects you” or “C would like to get to know you better.” Or the information could be unsettling or distressful, such as “C is quite angry at you” or “C is uncertain about maintaining a friendship with you.”

This triangle interrupts the rapport that might happen between B and C. It’s advisable for A to encourage C to bring the information directly to B.

The helper........

© Psychology Today