2 movies, not three, one this december and one next december. (i think)
They announced they're making three parts. Since Christopher Lee will play Saruman in the movies, and Saruman doesn't really occur in the book, I guess they will also show what Gandalf does when he temporarily leaves the dwarves (hunting the Necromancer). That could explain the three parts.
The simarrilion is five books only, and they could easily add the first two into the quenta simarrilion.
I got a one book version. But the point is not the amount written, it's the amount that is portrayed. You couldn't make reasonable movies out of the Silmarillion. What you could do is a series of cartoons, that could work.