In the 3rd grade, students are becoming proficient readers, who can read for entertainment, learning, enjoyment, and more.
Part of becoming a good reader is reading books at your "just right" level most of the time--books that are not too easy and not too hard.
Good readers read all levels of books, but they usually try to read just right books. If we read easy books all of the time, we won't learn how to read more difficult text. If we read challenging books all of the time, we get too discouraged and don't enjoy reading. Most of the time, we try to choose just right books.