Book 3 finale was VERY, VERY similar to ATLA finale.
In the epic scope and score, in the last epic battle (two flying supermen duking it out among phallic rocks formation), in the general impression (ending is very sad and the canon is irreparably changed again).
Unfortunately Zaheer as is the case with all villains in Korra went from somewhat interesting conflicted character to cackling madman by the end, which somewhat cheapens the overall impression of the season.
Still, a very strong finisher. Season 2 had Origins two-parter which i felt was on par with ATLA S3 finale, and book 3 of Korra ends with the finale of the same caliber.
Brilliant show, guys.