I've been looking for more documentation about what other cultures have traditionally thought about third nipples and found this one:
The occurrence of supernumerary nipples has been documented since Roman times and featured in legends and ethnic mythology prior to that time. Supernumerary nipples, and particularly polymastia, were attributed to increased femininity and fertility. Ancient artists depicted the goddess of Artemis of Ephesus and the Phoenician goddess of fertility, Astrate, like other ancient deities, as having row upon row of breasts on their chests. Supernumerary nipples in men were a sign of virility and endowed them with divine powers. Nowadays, film stars expose their extra nipples in the cinema with this same effect.
Now other than Mark Wahlberg, I can't really think of any other film stars who have displayed a third nipple in the cinema. And his virility and "divine powers?" I really can't say. One thing I do like, however, is that the website quoted above - emedicine.com - interjected a bit of humour in their medical information.