They have a wide range of reading levels.
Although most children can read fluently by third grade, the difference between the best and worst readers is extreme. It’s difficult for third grade teachers to determine an appropriate curriculum for reading, and schools generally allow children to choose books for themselves from the school library.
What you should do: Help your child find books that she can read fluently, regardless of grade level. Find topics that your child enjoys. Ask her to read aloud, so you can gauge her reading skills. Make sure she is reading at home, without distractions, for the entire time she is required to by her teacher.