For a brief period, Adobe provided a special version of CS3 that did not require activation for existing customers. However, those download links and replacement serial numbers have also been removed from Adobe's site.
For users running legacy hardware, Photoshop CS3 was specifically designed for the following environment: Mac OS X v.10.4.8 through 10.5 (Leopard). Processor: PowerPC G4, G5, or multicore Intel processor. Memory: Minimum 512MB RAM (1GB or more recommended). free download adobe photoshop cs3 for mac os x 10411 97 link
If you are looking for a way to use Photoshop on an older system like Mac OS X 10.4.11, The Status of Photoshop CS3 Downloads For a brief period, Adobe provided a special