What product is needed for a CS3 upgrade?

If you faithfully registered all your products, you might find your old serial numbers online.

Please log-in to your account below.
https://account.adobe.com/products

If you didn't keep your serial numbers, there's nothing anyone can do because 1) this is a user-to-user forum and 2) All Creative Suite products are unsupported.  Use at your own risk.

eBay is a pirate's den.   You would do well to steer clear of all software offered from auction sites.  The likelihood of receiving bogus, hacked or malware infected products is very high.   It's not worth the risks to you or your computer.  See Adobe Genuine Software https://www.adobe.com/genuine.html

If you have a nice old system, or a lot of luck, you can get CS 5.5 running (with the disk and serial and the one serial you upgraded from). CS3 is useless, except that you need to keep that serial. It's useless because, if you install it, you can't use it until it's activated, and the activation was switched off: gone forever.

The CS versions were discontinued after CS6.

You cannot install CS applications now.

You will have to get a subscription of CC to use the applications of your choice.

Good evening jannerick.
CS products were discontinued 8 years ago in CS6, so there is no version upgrade at present.
You need to purchase a Creative Cloud product.

In addition, the CS3 activation server has been stopped for several years, so if you do not have the activation avoidance installer, you cannot install it now.

In any case, installing and using the CS product in the current environment (from now on) may not be possible even if it is an own risk, because security patches may be stopped or support may be terminated. It is reasonable to think that it is impossible.