@itsBlurry01 The serial number is the CD Key, which you can see in Steam under Manage. If that key isn't working, please make sure that you're writing it correctly, for example that you haven't mistaken an o for a zero or vice versa.
If that doesn't help, are your Steam and EA accounts linked? It's possible that Sims 3 was already added to your EA account due to the link, although I don't know for sure that this happens or that it should.
Otherwise, you'll need to contact EA customer support. Take a screenshot of your Steam library with your username and the CD Key both showing. You can upload the screenshot when you submit the help request. I would strongly recommend using live chat rather than email. If you don't see the option, try this link:
Make sure you're signed into an EA site (this one is fine) before clicking the link and that the location in the lower-right corner of the browser window is set to U.S. or U.K.
