It has been reported that the first four episodes of Legend of Korra's Season 3: "Change" have already been completed. Bryan Konietzko (co-creator of Avatar) shared this news on his Tumblr blog around mid-November when he met with three of his friends from Studio Mir (Korean animation company), who were visiting Los Angeles at the time.
While we do not have an exact release date as of now, we know for sure that Book 3's wait will be much less than Book 2.
Additionally, some sources even anticipate that Book 3 will be premiering in 2014. While this is not officially confirmed, it seems quite likely.