bob_boh, good evening.
Due to a policy change, it appears that the download links for older versions of CC products, including CS products, have been removed at the same time.
https://helpx.adobe.com/support/programs/support-periods.html
If you haven't already downloaded the installer, you're out of luck, but there's no way to get it.
CS3 products are very old products. There is no security concern due to the suspension of the update, and there is no guarantee of operation in a popular OS environment with the current PC (especially OS 32-bit support was discontinued in macOS 10.15 Catalina) (It does not work 100% because of the relationship)
It might be a good idea to take a look at the Photography Plan, a CC product.