Citation needed. I have an AMEX in my wallet and it does not have any codes on the back, nor have I ever been asked for one.
I don't recall ever being asked for the three-digit code on the back (like I am with most VISA/MasterCard credit cards). It's always the four-digit one on the front.
Some discussion, which doesn't shed a whole lot of light though: https://insideflyer.com/forums/threads/what-is-the-3-digit-c...