Third wheel
Angela: "You two go on ahead without me. I don't want to be the third wheel."
安吉拉：你们两个去吧，我可不想当电灯泡。
The "third wheel" refers to the person who is the outsider when there is a group of three.
中文意思：第三者；电灯泡。
Two's company three's a crowd
Angela: "I hope he does not come over to our table; two's company; three's a crowd."
安吉拉：我希望他别来我们桌，两人成伴三人不欢。
We say "two's company; three's a crowd" because sometimes two people are having fun together and would not have as much fun if a third person joined them.
中文意思：两人成伴三人不和。