From screenrant.com -
It?s one thing to adapt a three-volume piece of material into three separate films, but a roughly 300-paged (depending on the edition) tale divided up into three movies? Jackson admitted, ?We were originally doing two films,? but pointed out, ?It?s a misleading book. It?s written at a really breathless pace. Pretty major events of the story are covered in two or three pages.? He even goes as far to liken it to a child?s bedtime story. While this might make it sound as though The Hobbit is even less suitable for a three-film adaptation, Jackson notes that making this film called for some serious character development and conflict.
Armitage used the dwarves as an example. ?The dwarf characters, for instance, in Tolkien?s book, they?re very thinly sketched and actually they?re a bit of an amorphous group whereas [in the film] every single dwarf you will get to know throughout the course of this journey.? He continued, ?As you?ve seen from the first film, the grand themes are feathered into the texture of it and in order to do that fully and allow each character to have their moment and to play their part in those themes, you will absolutely need three films to do it properly.?
McKellen was a bit more blunt about it, explaining, ?Anyone who thinks Peter Jackson would fall for market forces rather than artistic imperative doesn?t know the guy and hasn?t examined the body of his work.? He joked, ?If we just had made one movie of The Hobbit, the fact is that all the fans, and I?m thinking of the eight, nine, ten-year-old boys and girls, they would watch it 1,000 times. Well, they?ve now got three films they can watch 1,000 times.?
In all seriousness, Jackson pointed out, ?We also adapt the appendices from The Return of the King,? which tacks on about another 100 pages of material. The goal was to use that material to expand The Hobbit while also connecting it to The Lord of the Rings.?