How common is it to have three nipples? Well apparently MOTHER NATURE HERSELF has three.
Of course, I'm only going by Wiki-freaking-pedia here, who have reproduced this image of The Temple of Nature:
The image itself is the frontispiece for Erasmus Darwin's book "The Temple of Nature". (This book, published in 1803, predates his more famous grandson's book on evolution by at least 57 years. Check it here.
At any rate, Mother Nature with a superfluous nipple should be a lesson to us all. Three and proud, people, three and proud.