I'm certain that hunts for mentions of third nipples wasn't the intended purpose of the project, but sometimes greatness just happens.
I found this short mention of the subject of superfluous nipples here:
From "Anomalies and Curiosities of Medicine by George M. Gould and Walter Lytle Pyle"
Polymazia (or polymastia) sometimes seems to be hereditary. Robert saw a daughter whose mother was polymastic, and Woodman saw a mother and eldest daughter who each had three nipples. Lousier mentions a woman wanting a mamma who transmitted this vice of conformation to her daughter. Handyside says he knew two brothers in both of whom breasts were wanting.
Supernumerary nipples alone are also seen, as many as five having been found on the same breast. Neugebauer reports eight supernumerary nipples in one case. Hollerus has seen a woman who had two nipples on the same breast which gave milk with the same
regularity and the same abundance as the single nipple. The Ephemerides contains a description of a triple nipple. Barth describes "mamma erratica" on the face in front of the right ear which enlarged during menstruation.
That's what you get for being curious. Now you can be thankful that you don't have a nipple on your face too. And if you do, well, "Mamma Erratica" is a pretty cool nickname.