Think of a bicycle: it needs two wheels to function properly. Adding a third wheel makes the bicycle unstable and awkward to ride. Similarly, a third wheel in a social situation disrupts the dynamics, often making others feel uncomfortable.
Here are some common ways the idiom is used:
* "I didn't want to be a third wheel, so I went home early." This means the speaker felt out of place in a romantic setting and left to avoid awkwardness.
* "They're going to the movies together, and I'm definitely not being a third wheel." This shows the speaker is aware they're not wanted in the situation and won't force themselves into it.
The idiom can be used in a playful or humorous way, but it generally carries a negative connotation. It suggests someone feels excluded or unwanted in a situation, usually because of a romantic dynamic.