If there was a third act scripted, it wasn’t in the script they used for the book.
The only major scene omission is the birthday party. The overdubbed scene of the kids at the tree in the rain is a little bit longer in the book, but they did a decent job with it in the finished film.
For those interested, I did a writeup about the book a while back: http://www.imdb.com/title/tt0089114/board/nest/39489468?p=1