The Secret originates from the law, either as attributed to a profession, then it will be called Professional Secret or as attributed to those who exercise a state authority. It will then be designated as Secret of Function.
Confidentiality emanates from private law, mainly as the result of a contractual clause or of a contract. One, several or all the contractual parties commit to it and impose it on themselves.
Non-Disclosure is a product of common law , the legal meaning of which is not apparent. It means making arrangements for and giving instructions to those whose one is legally the Guarantor, such as employees, subordinates, children etc ... so that they respect a commitment to confidentiality.
Non-disclosure is too often mistakenly confused with confidentiality because many ignorant, professionals or not, write non-disclosure clauses with a text of confidentiality: the confusion is now part of tradition and supported by a doctrine soon majoritarian.