the third wheel第三者/電燈泡
--- The "third wheel" refers to the person who is the outsider when there is a group of three.
Example:
Billy: You two go ahead to the dinner without me. I don't want to be the third wheel.
比利：你們兩個自己去吃晚餐吧，我不想當電燈泡
ps你也可說the fifth wheel (By Billy)
