Third nipple is a miscellaneous mutation. It grants you the almighty abilities of having three nipples at your command.
Third nipple is located in the very bottom of rupture, in the science basement.
It allows you to tell someone's fortune by using 'fortune <victim>'.
You have three nipples, man. Psychic powers and shit.
Uh, none? Well, other than only being able to tell fortunes and taking up a mute slot. But c'mon! Three nipples!