Third wheel idiom
The word "third" can be subtituted by "fifth" without changing the meaning.
Used to refer to a person who is unnecessary in a group
Every time hanging out with Rachel and Ross, I feel like a third wheel.
When accidentally walking into a restaurant for couples only, I as soon realized I was a third wheel there.
I'm an introvert, so I feel like a third wheel among my classmates and struggle to find like-minded peers
Honestly speaking, nobody wants to be a fifth wheel.
This phrase may come from the idea of a two-wheeled trolley. when being added an extra wheel, that wheel seems to be spare and unnecessary.