Third Wheel
It's like a schematic of 3 wheels about the moment socialization:

-Third Wheel: biggest one, which consist in a group of people whom are you fond of, but not being very intimate.

-Second Wheel: small and inside the Third Wheel. People that is priority of your attention.

-First Wheel: smaller and inside the Second Wheel. Consists in something/yourself (or both) that is automatically a main concern.
Usually, for the majority of these wheels are like this:

-Situation: going out with friends/family and bf(gf).

Third Wheel: your friends/family

Second Wheel: your bf(gf)

First Wheel: yourself/your beliefs/your pride/your ego

-Situation: going out with friends.

Third Wheel: most of your friends

Second Wheel: your best friend

First Wheel: yourself/your beliefs/your pride/your ego