Adobe's web site warns users not to buy Adobe products on Ebay. Seems there are many selling cracked versions of Adobe's software there. If you buy from the wrong seller you can bet it will not qualify for any Adobe promotion.
No-one here is going to tell you it's OK to buy software off eBay.
Have a read through this article then make your decision.
PSCS3 came out a long long time ago. The odds of there being any legitimate new licenseable copies out there anywhere is pretty small. If you find a seller who wants to transfer ownership of a licensed, registered copy of PSCS3 and go through Adobe's registration transfer process, then OK. Good luck finding that.
CS3 is totally outside the upgrade loop. It's a dead end.
CS3 makes you eligible for the Photoshop CC + Lightroom bundle that is currently being promoted for $9.95 per month.
What's being unsaid here is that this is likely what would happen:
1. You would buy software from eBay, which would come with a serial number.
2. You would try to use that serial number to purchase the Photographer's bundle, and Adobe would almost undoubtedly not allow it, because the serial number is bogus or belongs to someone else.
So the short answer is: