Two's company and three's none.

A pair may enjoy one another’s company most when left alone. Adding a third person can change the encounter so much that it no longer feels companionable.

The saying expresses the perspective of people who want time together without an additional participant. It can also acknowledge that the third person may feel out of place.

Use it when a third person’s presence would interrupt a pair’s private conversation or time together. Because it can sound dismissive of that person, it is best used with care.