I bought a pre-owned battlefield 3 and the person I bought it from already used the code (game was from gamestop) what should I do?

I dont think that this is good practice from Gamestop and you will need to contact their customer services to get the problem fixed.

I did not even think that selling a pre-own copy with Battlefield is a verified practice, so I would question that with Gamestop.

you can try live chat or phone support for important problems https://help.ea.com/contact-us [help.ea.com] Under product select Battlefield 3, Under Category select Account/Registration, Under Platform select the platform.
You should see at the bottom Call Me Live Chat, Email Me. Select Either Call me or live chat (whichever one is available).

OP check your Gamestop receipt, they are giving out free online pass codes with used games after losting a California court case.  I'm in Virginia so they're not just providing codes in California.  I purchased a used copy of Uncharted 3 and they provided the code free of charge 3 days ago, if the code is not on your receipt take it back to Gamestop and ask for it. 

Failing that you may have to purcahse a new online pass for $9.99 since technically EA isn't mandated to provide a second code for free to a disc.