Re: Command and conquer Tiberium wars
If you have the code you can redeem it on Origin to permanently link it to your Origin account.
http://help.ea.com/en/article/origin-code-redemption-faq/
Then you can just download the Origin version of the game through the Origin desktop client 🙂
I'm fairly sure C&C3 exists as separate product in the Origin store, so you should be able to redeem a C&C3 code.
