the third wheel

The "third wheel" refers to the person who is the outsider when there is a group of three. — третий лишний

Example 1: "You two go on ahead without me. I don't want to be the third wheel."

Example 2: "Missy and I were going to go to the dance with another couple, but when I got sick she decided to stay home also. She didn't want to be the third wheel."

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An analogy can be made to a two-wheeled bicycle or cart; adding a third wheel would not be helpful and would limit mobility. To be the "third wheel" is to be the extra, unnecessary person in a group of three people. (GoEnglish.com)

See also
[two's company, three's a crowd]
[play gooseberry]
[третий лишний]
[быть лишним]
the fifth wheel
tag along