As a matter of fact, there are many other users who still use previous versions of CS3 because those versions simply work normally and because the upgrade price is sometimes way to high. However, if you didn't deactivate the serial, then I recommend contacting the Adobe Support team because the product you have purchased is still yours and you are entitled to use it even if the serial number has been lost or stuck on a machine. Adobe should be able to unlock it by replacing it and blacklisting the other one.
Use the following link to reach Customer Care: https://helpx.adobe.com/contact.html
Provide all the data and they should replace the serial number or provide a solution to this issue.