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

It won't install because its used. EA most likely won't give you a new code, they'll just tell you to get a new copy. One copy per computer and thats it. The previous owner likely registered the game when he played it so it can never be installed on anything else but the computer with his account again.

You essentially just bought a rather expensive coffee cup coaster. ☹️