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 All the games are available on Origin as part of The Ultimate Collection, the EALA games are also available individually on Steam (though if you get TUC you don't want to buy them twice), and the Remastered Collection (remasters of Tib Dawn and RA1) is available separately (not part of TUC) on both Steam and Origin.
You can download the multiplayer portion of Red Alert 2 for free from CnCNet somewhere, but not the campaign.
Only the first three games (Tiberian Dawn OG, Red Alert 1 OG, Tiberian Sun) are freeware.
