The most reliable place to secure a copy is through GOG.com , which offers a DRM-free version of the game that does not require an active online connection to play.
If you purchase a digital key from a third-party seller, it is typically redeemed through the GOG platform: Empire Earth 3 - PC : Video Games - Amazon.com empire earth 3 cd key
Second-hand retail copies can sometimes be found on sites like Amazon or eBay, though these require a disc drive and the original printed CD key for installation. How to Redeem and Activate Your Key The most reliable place to secure a copy is through GOG