This was revealed yesterday from a James Cameron interview with 20 Minutes. From Gizmodo:
Another article about this news via Murphy's Multiverse.
Speaking with the French news outlet 20 Minutes (via Culture Crave), James Cameron revealed Avatar 3 will introduce a villainous group of Na’vi called the Ash People.
James Cameron: "I want to reveal the Na’vi from another angle because I have only shown their good sides. In the early films, there are very negative human examples and very positive Na’vi examples. In Avatar 3, we’ll do the reverse."
Another article about this news via Murphy's Multiverse.