Harcourt Math Grades 1-6 http://www.harcourtschool.com/menus/math2004/math2004_menu.html
AAA Math http://www.aaastudy.com/grade3.htm
A+ Math http://www.aplusmath.com
Up To Ten http://www.uptoten.com/kids/kidsgames-home.html
Funbrain http://www.funbrain.com
I know that http://www.iknowthat.com
All kinds of math practice to cover third grade skills! 3rd Grade Math Practice - Money