Lost sims 3 pets registration code

@graciecake1  If you still have access to the computer where your game was originally installed, you may be able to recover the code.  Even if you uninstalled TS3, the code may still be accessible unless you also deleted it manually.  Here's how to recover your TS3 codes:

Windows:  https://sims3.crinrict.com/en/2013/09/how-to-find-your-serial-number.html

macOS:  https://bluebellflora.com/finding-your-serial-key-using-terminal/

Still though, if you want to install via Origin, you'll need access to the account where the game was originally registered.  EA doesn't allow a code to be used in two different accounts, even by the same person.  You could try entering your various email accounts in Origin and clicking "forgot my password"; the correct address will receive a password reset email from EA.  Otherwise, since the code was already used, there's unfortunately not a whole lot you can do.