While you might find a serial number online, the shutdown of Adobe's activation servers makes using the original CS3 disc very difficult. Your best bet is to look into your Adobe account for "Non-Activation" installers or move your projects to a modern, supported alternative.
Fireworks CS3 was discontinued in 2013 and has reached its "End of Life" status. This means Adobe no longer provides updates, security patches, or technical support for the software. Activation Servers Shut Down adobe fireworks cs3 serial number