Welcome to Gaia! ::

Hopefully to curb any misinformation, here is everything we know about Book 3:

-Book 3 will be the first 13 episodes of season 2.
-Studio Mir will animate ALL of Book 3 (and 4).
-Metalbending Police chief Lin Beifong and her family will play a large role; "We’ve got great stuff planned for Lin and her family. It’s not right away, so don’t be mad if it doesn’t happen right away. ----Book 2, for the Lin fans…she’s not very heavily featured in Book 2, BUT, very heavily featured in ----Book 3" - Bryke during a book signing. source
-Grey DeLisle, who voiced Azula in ATLA and the scorpion spirit in the Korra Book 2 finale, will be voicing a "bad girl" in Book 3. This character will NOT be a firebender (and not necessarily evil or a villain). source
-This scene with Asami in some kind of boat will happen:
http://25.media.tumblr.com/6607db7f2e6a622b168825c345b11d49/tumblr_mmxkrpAcrh1rptk5lo1_1280.jpg source

Book 3 production progress:

-The first four episodes’ “Take 1 animation” has been completed. source
-Bryan Konietzko: “The premiere date hasn’t been pinned down, but we are on track to airing Book 3 with MUCH less of a wait than the unfortunate one there was before Book 2 aired.” source

Unclear information/rumors:

-The title of Book 3 has been reported to be Shogun or Connections. These are both almost definitely not true.
-Korra and Asami could bond and become better friends. source
-Bryan has confirmed the current reigning monarch of the Earth Kingdom is an Earth Queen. -Given that Book 3 is supposed to focus on Lin, many people have interpreted this together as an indication that this Earth Queen will appear in Book 3.

Furthermore, production on Book 4 is underway as well.
Book 4 is the confirmed end of the show, with 13 episodes.

-Jason Isaacs (Lucius Malfoy - Harry Potter, Zhao - ATLA), Jason Marsden, and Anne Heche have been added to the cast in at least one scene with Jinora. source
-There is at least one scene with Katara and Korra. source

Hopefully with the shorter wait for Book 3, we will be getting more and more information soon!