It's way too early to know whether Nickelodeon will decide to order a third series. It depends on how many viewers will watch the Legend of Korra to the end, and if there is a great amount of viewers. They'll also want it to be well received. The Legend of Korea is highly critically acclaimed, but ratings are もっと見る important to Nick. Here are some facts I got from Wiki about LOK ratings:
The premiere averaged 4.5 million viewers ranking it as basic cable's number-one kids' 表示する and 上, ページのトップへ animated program for the week with total viewers. The Legend of Korra also ranks as the network's most-watched animated series premiere in three years.
Book One: Air drew an average of 3.8 million viewers per episode. This was the highest audience total for an animated series in the United States in 2012.
Book Two: Spirits premiered with 2.6 million viewers. Suggested explanations for the reduced number of broadcast viewers were: the long period between seasons, a change in time slot (Friday evening instead of Saturday morning), the increased availability of digital download services, and generally reduced ratings for the Nickelodeon channel.
Book Three: Change aired on short notice in June 2014 after Spanish-language versions of some episodes were leaked on the Internet. The season premiered with 1.5 million viewers.
Korra, Aang, Kyoshi and all the other avatars are the continuous incarnations of the first avatar, so even though Korra has ロスト connection to them, they are all still her past lives. Apparently she has started a new cycle of avatars, and she will continue to be reincarnated, since she recently went into the アバター state - so Raava is still infused as part of her. The future avatars will still be incarnations, so the first アバター will continue to live on. If I've made a mistake, please correct me anyone. xD