If you still possess your original CS3 proof of purchase, your best course of action is to contact directly to see if they can provide a legacy download link, though they may ultimately recommend upgrading to a supported version.
Because the original servers no longer exist to "handshake" with your installation, standard activation methods fail. The Official Solution (Legacy Support) adobe cs3 activator
Adobe CS3 Activator
If you still possess your original CS3 proof of purchase, your best course of action is to contact directly to see if they can provide a legacy download link, though they may ultimately recommend upgrading to a supported version.
Because the original servers no longer exist to "handshake" with your installation, standard activation methods fail. The Official Solution (Legacy Support)