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。

I know I'm the third wheel here.

我知道我才是电灯泡。

I could not tell who was the third wheel.

我没看出来谁是电灯泡。

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。