一不小心就成了 a third wheel

　　『 a third wheel』

　　电灯泡，当下不受欢迎的人

　　释义：

　　a third wheel or a fifth wheel

　　见柯林斯英语词典

　　例句：

　　For some showbiz couples there's a third wheel in their relationship - the meddling mum.

　　I just wanted to feel like part of the family instead of a fifth wheel.