Install Photoshop CS 3 on Mac

Forget about your copy of CS3 and your serial number. They won't work because Adobe has retired the CS3 activation servers.

You now need to download a new, non-activation copy of CS3 and a new, non-activation serial number.

See Activation or connection error when starting CS3 or Acrobat 8

Also, to install on High Sierra, see instructions at Installing Creative Suite on macOS 10.12 (Sierra)​

Note: since CS3 is so old, there's a strong chance that it may not install or run correctly on High Sierra. If that's the case for you, your remaining option may be to subscribe to the Adobe Cloud.