third wheelphobia
Someone who is so afraid of being a third wheel that they leave as soon as a couple or two people start talking. Often they develop this phobia when being forced into a situation where they are a third wheel or someone's make out bitch.
person 1: Why did Lindsey just leave?
person 2: Oh she's had third wheelphobia ever since Natalie used her as her make out bitch.
person 2: Oh she's had third wheelphobia ever since Natalie used her as her make out bitch.