CS3 serial number not accepted

(Mac El Capitan V. 10.11.6) Have re-installed my CS3 design standard. Issue is that PS & AI in this suite downloaded from discs and appear in Applications folder in their entirety, but when I go to open to start work, they ask for SN. When I enter the number, the message: "The serial number you entered is not valid. Click ok to retry entering the serial number." I have done this dozens of time & same message comes up. Indesign CS3 launched fine. It never asked for SN. I use stand alone CS4 for Indesign & no problems re-installing it.