User blog comment:Draevan13/Differences between books and series/@comment-4463619-20120730054117/@comment-2065721-20120730123326

It only the first half of book three they use for the third season. But GRRM has revealed that certain events from the other half is going to be end of the season. So they are gonna move some things. The rest of book three, possibly also from book four/five, is going to be the fourth season.

They have revealed that Theon is going to be in thiss season, which he aren't i the book, he has some flashback in book five, so they can use the flashback scenes.