I have thirty handwritten pages and more in my head but will they come out? No. Why not? Can't decide on the beginning... Have a secondary character waiting on #1 or have #1 start? When in doubt: Ask the character! After all, it's their book. I'm just chronicling the adventures, the angst, the passions, the catastrophes. The writer needs to trust the characters. Often they know more than the author gives them credit for, especially since it's the third book and they've done a fine job in 1 & 2.
Everything is open to change. Nothing is set in stone, and if it is, the chisel can plane across and smooth the slate. What if no one likes it? I've heard from several who want more, they can't be the only ones who like the writing. (They're not even family so they don't have to be nice!)
Keep writing, keep track of who's doing what to whom and things can move if they have to, that's the way 1 and 2 were done. It works! Buzzduck! (eh hun? lol)