I'd be interested in error messages, if any, in the logs (run Console).
One admittedly complex possible solution would be to install 10.5 on an external drive, boot from that drive, and install CS3. Or use Firewire Target Hard Drive mode using two Firewire enabled Macs (one running 10.5) to install CS3. I don't know if that would work or not, but if it can be installed successfully in 10.5 but not in 10.6, it might work.
Good luck. Hope you find a solution. Rolling back to 10.5 might be a simpler solution.