I'm known to be a third wheel.
Example:
Trio #1 - My friend Andrea. My friend Hailey. Me. Andrea and Hailey are both lesbians and they're dating each other. They spend so much time to together, even when I'm around, that it's as if I'm simply invisible.
Trio #2 - My friend, Jessica. My former friend, David. Me. Jessica and David both did like each other, but never were dating. When David joined the trio of friends, it was like I was suddenly just a stranger.