I just had a quick look around, to see whether anybody else is having problems with CS3, and found this official-looking link:
https://helpx.adobe.com/creative-sui...acrobat-8.html
Apparently even though CS3 is not a subscription service, it does use a certificate authentication step, for which it needs to talk to an Adobe server. If that server is not avialable (or, "is aging and had to be taken down", like a dog), you can't use your software.
The page also says you can "
Receive a new serial number and installer", presumably this will work without a server. But you have to create an Adobe ID and request it...
Pressing thumbs you can use your files again.