When I enter my Sims 3 product code it says ''This code has already been used''.

You're probably having trouble reaching a game advisor since it is Sunday.  I know they don't take phone calls then and their live chat support is minimal on Sundays.  

As for your game, I highly doubt you'll be able to get another code.  Newer versions of the game come with one code that can only be tied to one Origin/Sims account and the installation requires this one time code.  This is to prevent people selling their games without EA making any kind of profit.  Sorry.  

You definitely try returning it.