Thoreau had three chairs in his cabin by Walden Pond: “One for solitude, two for friendship, three for society.”
I think Thoreau could have thrown out the third and been just fine.
You can talk to the third person later, when it’s just you two.
Imagine a dance, with two partners the interaction is clear. With three, one is left out.
In conversation with two, it’s obvious whose turn it is to speak and whose turn it is to listen.
With three, two listeners and a speaker. Toes get stepped on. And even when a speaker does take the stage, she averages her words for two, instead of talking directly to one.
If you find yourself to be the third, just be a fly on the wall and listen. So the interaction between the other two is still clear. They’ll respect you for listening, and ask for your opinion when they need it.
Source: Henry David Thoreau, Walden.