You can choose one student at a time to read a paragraph aloud. After the student reads, you can ask the student why she thinks the author wrote the story or to explain the purpose of the paragraph. Give the student time to come up with an reason and explanation of what they read. This will help you to learn more about your students and whether each student is understanding what is being read.
You can allow your students to teach a comprehension strategy to your classroom or ask a lower-level teacher to give your students permission to teach her younger classroom. A couple of comprehension strategy ideas that your third graders can explain in front of the class are: how a reader makes a connection between what he reads and how it relates to everyday life or fiction (movies), visualization (creating pictures in the mind) and the importance of asking yourself questions when reading. You want your students to expand on these strategies.
If there is not a book club at your school, you should talk to the principal about starting one for third graders where each student can discuss her favorite books among her peers. This will be a time where a student can discuss what she thinks is the main idea in a story, discuss characters and how the story relates to her life.
You can have each student color-code his textbooks (if allowed) or novels when he comes across a word or key information. Students can use different color highlighters or markers to underline information, which should help them to remember the concepts and make it more accessible to go back to later.