As is the case for any sites visited by your child, parental guidance is recommended for each of the following. These suggestions are intended to supplement students' at home studies.
class brain
This is a good interactive site. The site includes information about each state (great for Social Studies) and various games for math and vocabulary building.
TAKS practice
Choose TAKS Released tests for 3rd grade. The site provides practice tests for Reading and Math.
AAA math
Another good interactive site to practice math.