Nah, I wouldn't worry about that. But I don't want to be the third wheel.
我倒不担心这个，我也不想当你们两个人的电灯泡。
凯莉日记 第2季
I know I'm the third wheel here.
我知道我才是电灯泡。
第三季
I could not tell who was the third wheel.
我没看出来谁是电灯泡。
破产姐妹 第2季
She doesn't have to know she's the third wheel.
不必让她知道她是电灯泡的事实。
英语百科
Third wheel
（重定向自The Third Wheel）
The term third wheel is a corruption of fifth wheel, which refers to someone who deters the socialization of a couple, perhaps when being invited out of pity or through a feeling of duty.