"When we hit our lowest point, we are open to the greatest change", a fitting title for Book 3 IMO.

K112 Screen

We know this thanks to an interview with Mike and Bryan done by WSJ, right now only a new post referencing this is up, the full interview will be up later on. In addition to the confirmation that Book 3 is called "Change", Mike had this to say.
“It’s all about Korra dealing with the change of the world in this point in time. At the end of Book 2 she says it’s a new era. And it really is and we’re going to show how Harmonic Convergence and her battle with Unalaq and Vaatu kind of shifted the world a bit.” “We don’t have an airdate yet. We’re already working on the first few episodes of post-production for Book 3. So that’s all going very well.”
Makes sense to me, spirits and humans being allowed to freely enter each other's worlds will definitely lead to a lot of change. I am excited. Thoughts?