Downloading and using the software without a license is a violation of Adobe's copyright. While pursuing individual users is rare, it is technically illegal and, in some regions, can carry significant penalties.
Adobe Photoshop CS3 is officially a discontinued "legacy" product. There are no legitimate sites offering a "free" full download of the software today, as Adobe has retired the activation servers for this version. The Reality of Photoshop CS3 on Mac OS X 10.4.11 free download adobe photoshop cs3 for mac os x 10411 97 link
Adobe Photoshop CS3 is no longer available for free download from official sources, and for this version in 2017. This means even if you have a legitimate installer or original discs, you cannot newly activate the software. Current Status of Photoshop CS3 Downloading and using the software without a license