三人行必有一是电灯泡
[sān rén xíng bì yŏu yī shì diàn dēng pào]
'Among Three People Walking Together, There Is Bound to Be One Extra Person as the Third Wheel'. This phrase comes from a popular Chinese idiom that usually means when three individuals get together romantically, one ends up feeling superfluous and unwanted, like a lamp left shining unnecessarily.