When a couple becomes bored with their relationship and begins to constantly "hang out" with a "friend" together and thinks this friend is the coolest thing ever. Both people in the relationship crush on the friend, usually unknowingly at first. A third wheel crush requires at least one bi or gay person on the couple's side.
As Kit Porter from the L Word says, it's dangerous business.
If you identify yourself as the third wheel crush in a friendship, get out of it fast. It usually leads to one of them cheating with you, a threesome that goes nowhere, or lots and lots of screaming and blaming you for destroying what they had.
Friend #1: "John and Mary text me every day asking if I want to go over to their house and hang out. And then each of them will invite me to go eat out without the other being there, like they're taking me on a date. If they weren't together I'd think they both had a thing for me."
Friend #2: "You're in the middle of a third wheel crush, man. Nothing good can come from this."