The story will follow an adult Hiccup in his adventures in a world without his best friend and dragon Toothless, and the disappearance of all the dragons in the world.
DeBlois then said that the movie will be opening up five years later where the movie's hero Hiccup is "at a different rite of passage."
Hiccup had been named the village chief after his father Stoic's passing in the previous film and has to struggle with feelings of inadequacy after taking over. He also has to face the fact that the dragons have all disappeared, including his own dragon Toothless
All the dragons will die. Every one of them!
Or, at least, they will disappear. The very first lines of the first book read:
There were Dragons when I was a boy
By this time, Hiccup is an old man reflecting back on his life, and his adventures with Dragons.
In the book, the first line reads: "There were Dragons when I was a boy."
The narration is done by Hiccup when he becomes an old man. He is reflecting on his life as well as his adventures with Toothless and the other dragons.
Well, this doesn't mean that all the dragons will be killed by the evil forces, but there's always the possibility of that happening. Other possible reasons could be that the dragons will choose to migrate to some other place. But that's doubtful because seeing the Vikings and the Dragons live together in peace and harmony will be the ending that fans of the book and movies would want to see.
Read more: http://www.christiantoday.com/article/how.to.train.your.dragon.3.plot.release.date.moved.major.spoiler.revealed/42699.htm#ixzz3QCVuWqaG
