Wiktionary
n. 1 (context idiomatic English) A person or thing that serves no useful purpose. 2 (context idiomatic English) An unwanted third party accompanying two people on a date.
Wikipedia
"Third Wheel" is the third episode in the third season of the television series How I Met Your Mother and 47th overall. It originally aired on October 8, 2007.
Usage examples of "third wheel".
When a third wheel was fitted with a stub axle and slid into place in the forked lower end of the post, the test stand looked remarkably like a three-wheeled, steerable, steam-engine-powered platform that was mounted on legs.
This wheel was connected to the second smaller wheel by a horizontal belt, and that second wheel, in turn, was connected to the third wheel by a vertical belt that ran up to the top of the tower.