Hi Shawn Manwaring,
Welcome to Microsoft Q&A forum. I'm happy to help.
Please note that this is Microsoft Q&A forum, a community to share knowledge and ask questions about Microsoft product and services, and SimCity 3000 is not a Microsoft product.
If you bought SimCity 3000 from a platform such as Steam, you would get digital license and don't need product key to play.
If you bought the key to redeem SimCity 3000 or key of SimCity 3000 directly and forget the key, please contact the provider.
We're sorry for any inconvenience may have caused and thank you for your understanding.