I don't want to be a third wheel.
be a third wheel直译过来就是：“做第三个轮子”，自行车上装第三个轮子表示累赘，引申到男女约会时表示：“多余的第三者，当电灯泡”。因此，当美国人说"I don't want to be a third wheel."时，他/她要表达的意思就是:"I don't want to be an unwanted third party on a date."、"I don't want to disturbyou two."、"I don't want to be the needless one."。
Terry: I'm going out with Iris tonight. Why not come and have dinner with us?
Benjamin: You two go on ahead. I don't want to be a third wheel.
①It's your private affair and I don't want to interfere.
②You two haven't seen each other for so long.
③You must have a lot to confide to her.
④Let's ask her out to dinner tonight.
1. wheel n. 车轮
The car wheels spun and slipped on some oil on the road.
The wheel of history rolls on.
2. interfere vi. 涉入
I wish everyone would stop interfering and just leave me alone.
The UN cannot interfere in the internal affairs of any country.
3. confide vi. 吐露
I knew she had some fundamental problems in her marriage because she had confided in me ayear earlier.
He confided to me that he felt like he was being punished.