I haven't seen this covered on the Odeck, but apparently a trailer for Book 3 Change has aired in the Netherlands. Even though it looks incredibly shitty quality, I buy that its real since Michael Dimartino has said that the first six episodes are done already, and there is only a bit of work left for Studio Mir to finish the book. Source for that here.
Hypable has some stills and gifs of the video. But I'm going to steal my two favorites.
Holy cow we got dragons, a shirtless Tenzin, the White Lotus dudes (who are rumored to be the new bad), and Korra still kicking ass. I'd say we are close to getting a trailer over here in the states, so be on the look out.
Also Michael Dimartino regularly updates his blog with Korra stuff so check on it occasionally.