if it's still in the steam library try right click the game icon view CD key
edit to a link to an FAQ http://answers.ea.com/t5/The-Sims-3/FAQ-Steam-VS-Origin-VS-Discs/td-p/3135440
This does not work for me it’s too long!?!
@galaxycookie300 Do you mean that the code is longer than the Sims 3 Store or Origin allows? You want the option to "register a game," which requires 20 characters, rather than "redeem a code," which only requires 16. The first option is for the base game and its packs; the second is for bonus content.
I figured it out but when I click register it brings me to the “it looks like you already have an account...ect.” But I go put in the password and it doesn’t work then I click forgot password and it asks me to sing in again I have been trying to figure this out for almost a year now and at this point I’m ready to just give up...
@galaxycookie300 Try clearing all of your browser caches, and then see if you can reset your password. The store site is usually more reliable than Origin, but sometimes it acts up and doesn't cooperate for a couple of days. If you've ever signed in while playing, either at the launcher or Main Menu, it's also a good idea to clear your Internet Explorer cache, as TS3 uses it for some purposes.
clear IE cache: https://kb.wisc.edu/page.php?id=15141
