A third wheel is a person or thing that serves no useful purpose, or an unwanted third party accompanying 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。
所以说，哪天若有某对情侣邀你一起看电影，你完全可以说：You two go on ahead. I don't want to be the third wheel." "你们俩去好了，我可不想当电灯泡。"
For example:
A：Come with me to meet that young man tomorrow.
明天陪我去见见那个小伙儿吧。
B：Forget it. I don't want to be the third wheel.
算了吧，我可不想当你们的电灯泡。