If you think The Legend Of Korra Book 2 was already epic and mind-blowing, wait until you see the trailer of book 3! More sinister enemies, more potential allies, and more action from Korra awaits!
Book Three: Change is the title of the upcoming third book of The Legend of Korra, which will run for a total of thirteen episodes. Set a few weeks after the end of Book Two: Spirits, the nomenclature refers to the changes induced by the energy shift of the planet due to Harmonic Convergence and Korra's battle with the Dark Avatar.
As of May 8, 2014, the animation for Book Three has been completed by Studio Mir. Production work for Book Three was officially completed on June 3, 2014.
In this trailer, you will see Korra’s allies battling different anti-avatar factions. The action is incomparable with the first two books. See it for yourself:

As of the moment, Nickelodeon has already confirmed the following with regards to book 3:
It has been confirmed that more information will be learned about earthbending in Book Three.
Lin will play a larger role in Book Three than she did in Book Two, and more information about her family will be revealed.
Bumi will also be more heavily featured in Book Three.
It was revealed by Jeremy Zuckerman that Book Three will be more like Avatar: The Last Airbender.
On June 8, 2014, episodes three through six of Book Three were accidentally leaked on the Mexican Nickelodeon website, MundoNick.
The leaked episodes prompted the production team to release an official trailer for the third book earlier than originally planned on June 11.