APAD: Third wheel

A "third wheel" is a person, often single, who tags along with a couple or two close people in a social setting, feeling unneeded, awkward, or like a "fifth wheel". It describes an uncomfortable, often involuntary, position of being the "odd man out" in a romantic or intimate pairing.

It is a common, often temporary social situation. It can lead to feeling like an outsider, a burden, or an unwanted presence. 

The term stems from the idea that a tricycle only needs two wheels, making the third one unnecessary.

While often viewed negatively, some find being a third wheel fun, offering a chance to support friends or get to know new partners.