The Pearson Education website offers online reading tests for all grades, including third grade. The tests are free, and there are instructions regarding how to grade them in a similar fashion to the grading on an actual standardized test.
The tests on PearsonLongman.com use a combination of multiple choice, short answer and long answer questions. This provides practice for students in general comprehension as well as in processing what they have read, analyzing it and organizing their thoughts to answer questions regarding the passage. It also replicates a typical testing format that most children will be exposed to in their standardized testing.