CS3 Activation Servers were taken offline. You cannot use your old version & serial number. You must get a new CS3 installer and new serial number. See the correct answer in link below for details.
Re-activating CS3 (Updated)
And if you're on Mac Sierra, see the additional workarounds for installing legacy Creative Suite.
Installing Creative Suite on macOS 10.12 (Sierra)
Nancy O'Shea— Product User, Community Expert & Moderator
Alt-Web Design & Publishing ~ Web : Print : Graphics : Media