Give your third-graders journals that they can write in after a lesson in any subject. Supply them with open-ended prompts during journal time, such as, "I learned that ..." "One thing that surprised me was ..." and "One way I can improve is ...." Let students reflect on what they learned and what they still don't understand. Collect the journals and read them, which will allow you to identify areas of study that need more work.
Writing can be used to help students develop a deeper understanding of difficult concepts. After you have taught a scientific concept such as the greenhouse effect, for example, give students a few minutes to write a definition of it in their own words. Supply them with a few key words and their definitions to get them started. If students are stuck, allow them to look at a textbook definition before rephrasing it for themselves. Simply trying to articulate a difficult concept will help students assimilate the knowledge. Collect their writings or have them share with each other and compare definitions. Use this activity in science, math, music, history and social studies.
Even subjects that seem grounded in facts or numbers can be researched and written about. Assign your students research projects in subjects that are not normally associated with writing. For math, assign each student a famous mathematician and have him write a short biography. For science class, assign a famous scientist or scientific event, such as the 1986 Chernobyl disaster, when a nuclear plant in the Ukraine melted down. Have students research and write a report detailing important details. Make it clear that you will be evaluating their writing in addition to their research.
Even creative writing activities can help students learn about subjects other than English. In history, for instance, instead of just teaching students about slavery, you can have them write a narrative from the perspective of a slave in the 19th century. For science, have students write a story set 100 years in the future, imagining what society will be like. For math, have students write a creative essay about what the world would be like if math had never been developed as a concept.