While "The Legend of Korra" Book 2 awaits its September release, news has surfaced confirming that Book 3 will be animated by Studio Mir only.
As pointed out by Avatar Online, Studio Mir has been working with "The Legend of Korra" team since Book One, but various rumors continued to circulate that a Japanese animation company had been added to work on season 2 along with Mir.
Since then, it has been confirmed that Japanese animation studio Pierrot Co. has been working on Book 2 along with Studio Mir.
During the recent San Diego Comic-Con "Korra" panel, co-creator Bryan Konietzko confirmed that while Perriot Co. animated the Book 2 premiere, "Rebel Spirit," Studio Mir had animated the two-part episodes featuring the first Avatar Wan.
When asked by a "Legend of Korra" fan how much of Book 2 the company had helped animate, Studio Mir promptly responded, "We worked on half of Book 2."
But more recently, Studio Mir has confirmed on their Facebook page that unlike Book 2, "The Legend of Korra" Book 3 will be animated by them alone. Presumably, this also means that the final Book 4 will involve animation from Studio Mir as well.
And as much theorized, the involvement of two animation studios could be the explaination behind the Book 2 delay.
Perhaps with only Studio Mir working on "The Legend of Korra" Book 3, fans will not have to wait as long for the release.