my son also has the 3rd nipple under his left nipple. It was not discovered until puberty. because of his anxiety over it - and the puberty thing going on he mentioned it to our doctor and the doctor stated that it is not unusual, and will cause no harm.
I have also seen people born with bumps on their arms, that whne x-rayed shows a formation of bone - possible beginnings of another arm that did not complete.?

It is a miracle how our body functions - for instance look up what a Monster tete' a tete is in a medical encyclopedia.