Two’s company, three’s a crowd is used to say that a third person is not welcome when two people (such as two lovers) want to be alone with each other.

For example:

Well, I’ll leave you two lovebirds alone. Two’s company, three’s a crowd.

I was worried when the new boy moved into the neighborhood 

and started hanging out with John and his best friend—two’s company, but three’s a crowd.

Thanks for inviting me to hang out with you and Lucy, 

but I don’t want to get in the way. As they say, two is company, 

three is a crowd.

Can you excuse us for a moment? Two’s company, three’s a crowd, and we have something to discuss privately.

I think it’s best if I head out. Two’s company, three’s a crowd, and I don’t want to intrude. 

Tradução:

AI Overview

A expressão idiomática “two’s company, three’s a crowd” significa “um é pouco, dois é bom e três é demais.”