I think splitting Book 3 is a mistake. There are budgeted to be 7 books, and if you look at the history of television, even most great series only make it about 7 seasons. The Sopranos only went 6.
Book 3 is my favorite book, but I think they would be better off making the season 12 episodes than splitting it. Book 4 is slow and not much happens, but Book 5 has almost too much action and not enough character development. I figure they will combine the two, and split them chronologically into two seasons. Those who read the books remember that 4&5 were one book split into two, and Martin split the books by character instead of chronology, which turned into a giant plot mess that didn't make sense until the end of 5. I wonder if the decision to split Book 3 means that Martin is no where near done with 6 and finishing 7 in time for that season on HBO is a fantasy at this point.