MovieFan612

Correction: A third nipple is usually much smaller than the other nipples, several inches away and looks more like a mole. For example, Mark Wahlberg has a third nipple that looks like a mole. It is likely if Ross saw Chandler without a shirt on and saw anything, he assumed it was a mole and Chandler did not tell Ross what it was. Chandler likely mentioned it to Joey.

Kaltenmeyer

Correction: If Ross knew, then everyone else (except Rachel) would know too. If so, then why would Chandler be so angry at Monica for telling them. Obviously no-one knew about his third nipple because it was Chandler's big secret.