What does 'to be the third wheel' mean?

[Nenad11]

English Language Tests, Intermediate level

ESL/EFL Test #348 "English Slang Idioms (69)", question 9

"Are you sure you want me to come with you and your boyfriend to the lake? I don't want to be the third .........," Megan said to Alice.

(a) horse
(b) goat
(c) element
(d) wheel

English Language Tests, Intermediate level

ESL/EFL Test #348 "English Slang Idioms (69)", answer 9

"Are you sure you want me to come with you and your boyfriend to the lake? I don't want to be the third wheel," Megan said to Alice.

Correct answer: (d) wheel

Your answer was: incorrect
"Are you sure you want me to come with you and your boyfriend to the lake? I don't want to be the third goat," Megan said to Alice.
_________________________

Does to be the third wheel mean to be a nuisance
in a way?

[Mister Micawber]

Yes. 'Unnecessary'.
_________________
Native English teacher at Mister Micawber's