A third wheel¹ is a person or thing that serves no useful purpose, or an unwanted third party accompanying² two people on a date. Such a person is as useful as a third wheel on a bicycle.

Third wheel指不发挥任何作用的人和事物，或是在两人约会时陪伴在旁的一个多余的人，即我们所说的“电灯泡”。之所以叫third wheel是因为这样的人就跟自行车上放第三个轮子一样，一无是处。

For example:

Larry and Jenny asked me to go out for dinner yesterday, but I said no, because I didn’t want to be a third wheel.

赖瑞和詹妮昨天叫我跟他们一起出去吃饭，但是我没去，我可不想当电灯泡。