i'd definitely contact EA support. If it's telling you the code has already been used and you've downloaded it legit through Origin. You'll have to verify your purchase of the game to them (by providing proof of everything) in the hopes that they can give you a new serial code. For some reason it seems you might have been given a duplicate code.
if you registered the code in Origin first it auto regs on the Sims 3 site you use the same email and password if you use different ones you can run into that issues
contact CS via live chat
1.https://help.ea.com/en/contact-us/
2.Click on the orange NEXT button
3.Select a platform
4.Click on the orange NEXT button
5.Select a topic and describe your problem in a few words
6.Click on the orange NEXT button
7.Click Chat Now to chat with a Game Adviser
