The third faction in Tiberium Wars is The Scrin. GDI refers to them simply as The Aliens, while Nod calls them The Visitors. The Scrin campaign is unlocked once both the GDI and Nod single-player campaigns are complete.
The Scrin are the alien faction in C&C 3. Midway through the Nod and GDI campaigns, they invade Earth, for reasons unknown (most speculation has them doing it at the behest of another alien race, however). They are fully playable in skirmish matches as well as online play, and their arsenal features a mix of high-tech units and sheer power. It is quite probably the strongest late-game side in C&C 3, and features quite a few stunningly powerful units and structures that make their way to the battlefield once the game unfolds.
Scrin Faction Information:
C&C 3 Faction Information: