Mother nature's a mad scientist! Some humans and types of mammals born across the world today will find an interesting addition to their usual cache of two nipples on their chest. The proper term for this is 'supernumerary nipple', but seeing as that's very long to type out, this entry will prefer the term 'third nipple', though you may very well see fourth or fifth nipples in nature.
It is not really unusual for non-functional (or, vestigial) nipples to be present around the breasts, chests, necks, armpits, groin and other parts of male and female bodies. They're most likely to occur just about below where nipples normally are, but sometimes down near the lower abdomen, where other mammals have multiple nipples on their bellies. They can really pop up just about anywhere. Since our various ancestors may have had many nipples or more breasts, the trait sometimes comes back to the surface and causes an odd little accessory in our modern bodies. The third nipple can be blamed on, or credited to, a conspiracy between atavism and our particular path of evolution.
So it's there, you've got one. Now what? Well, if you are a male with a third nipple, your handling of it isn't going to be much different from the other nipples because all male nipples are essentially useless. But, in the opinion of this male Researcher who has a third nipple1, they look good.
The tragic thing about female third nipples is the segregation. The first two of them serve a purpose - to potentially give the milk of life to a baby through mammary glands. The other one is left aimless, without much purpose, save for a bit of hope. Sometimes, a third nipple can develop into a full breast, especially during pregnancy - which can be helpful for triplets. Ten breasts have been known to develop on a single woman. So there's always hope for the 'other' nipple on the one in 200 females who develop another nipple.
Some people have found a use for their third nipple, however. For those who are truly out of ideas, the third nipple can often be pierced. It can be used to gain attention in locker-rooms and sideshows. Some people find it to be an attractive physical feature, so it can be used to allure those sorts of folks (but only if you really want to entice a person who has a third nipple fetish2). Finally, it can be used to gain entrance to exclusive Triple-Nipple Clubs.
Incidentally, if you're thinking of forming a Triple-Nipple Club, you will need a way to distinguish a third nipple from a birthmark. If there's a noticeable bump around the centre of the thing, it's probably a nipple. If you're having trouble finding a bump, try putting some ice on it or going out in the cold. That should make the nipple come out to prove itself.
There are some reasons to not want a third nipple. You may be thinking of joining a Two-Nipple Club, or you may not want to be hunted as a witch3, so you might need to get your third nipple removed. There are ways to do this - simply visit a doctor and proudly proclaim that you want to join the ranks of bi-nippular society. Do not just lop it off with a hot knife yourself.
If the reader has a third nipple, he or she is in good company. Several famous people or characters have third nipples...
Krusty the Clown (full name Herschel Pinkes Remochel Krustofski), a character from The Simpsons has a third nipple on the right side of his chest, along with a pacemaker scar and a birthmark.
One statue of Artemis, the Greek goddess of virginity, depicts her as having dozens round objects on her chest - perhaps breasts (though they could also be bull testicles and eggs).
In the television show Friends, character Chandler Bing had his third nipple - which he had called a 'nubbin' - removed in an episode. Prior to his nipple downgrade, he claimed that when you pressed his nipple, a door opened to the magical land of Narnia.
Mark Wahlberg, also known as 'Marky Mark', star of such films as The Perfect Storm and Boogie Nights and a singing star in his day, has a third nipple.