The first three games have been released by EA as freeware, and these freeware versions come prepatched with fan patches, and so are preferable to the ones downloaded from Origin. Just make certain you follow these guides for each game to make sure you set them up correctly, or you may run into problems. The Ultimate Collection on Origin contains every C&C game, excluding the recent Remasters (the Remasters are available on Steam and Origin).
The following is a copy-paste I've made that covers how best to play the whole C&C series, it's a little more info than you were after but I think some of it'll apply to you The Ultimate Collection is the only legitimate digital source
