Run setup.exe as Administrator. Use any serial number from a valid CS3 era (numbers starting with 1130- for the Extended version). When it fails to activate, select "Enter Code Later" or "Trial."
If you are looking for the REQUESTED copy via the usual channels (archive.org, old warez forums, or specific "blaze69" tracker listings), be aware of the "Phoning Home" bug. You need a cracked amtlib.dll or the special Adobe tool to bypass the decommissioned servers. Adobe Photoshop CS3 Extended -REQUESTED- -blaze69-
To stop the 3D engine from freezing: Open Regedit > Navigate to HKEY_CURRENT_USER\Software\Adobe\Photoshop\10.0\ Create a new DWORD: OverridePhysicalMemoryMB Set value to 1024 (decimal). This forces CS3 to think you have 1GB of VRAM, bypassing memory check errors. Run setup