A mole often directly between the two real nipples. Completely useless as a nipple.

When you eat dirt for the first time and leave your family to build a shack in the woods

Used in the phrase "Don't take it to the third nipple!"
The "third nipple" is non-existant because it's one too many body parts we need, so it's used to refer to when someone is over reacting to something or is about to say something that's exaggerative (<-is that a word?).

something completely useless.

Also known as a third wheel... when a dating couple or almost dating couple decide to spend sometime together but one of the guys or girl friend is there. Sometimes this turns into Cockblocking.