I said on Thursday I expected, based on Sperry's original design plans dating from before the release of Tiberian Dawn, for the third side to be the Scrin. Apparently, it will at least be an alien race, though no name has been given yet. The Spanish website 3DJuegos
posted a preview of C&C 3, based off an interview with Mike Verdu, the Executive Producer. The interview, originally in Spanish but translated at RTSCommunity
confirms that the third side will be an alien race. The translation isn't perfect, but here's a (cleaned up) snippet for you:
Halfway through the combat between GDI and Nod, a new alien race will appear, attacking Earth to conquer it, and of course, fighting for the desired Tiberium. The player will be able to command this race in the campaign mode, although their campaign will be far shorter than the others (Approximately 35 missions for GDI and NOD, against 15 for the Aliens).
Like I said, to my knowledge, both from recent information and older information, this would sound right and make sense. I do expect there to be significantly less than the mentioned number of missions--remember that
Generals was also supposed to have more missions than it ultimately did. In the end, and this is a guess, I'd expect about 25-30 missions total for the game.
For those wondering, I think we'll find out a lot more about C&C 3 over the next two weeks. We'll probably see the Apoc/Predator thing resolved or advanced, and we'll get a lot more hands-on with the game. It should be fun!