Good homeschooling sites for third grade math lessons include Jumpstart.com, Homeschoolmath.net, and KhanAcademy.com. These sites include lesson plans, online video lessons and quizzes, and printable practice problems.
Jumpstart.com features educational games, lesson plans and curricula for parents teaching all grades. It also offers printable worksheets and a community for teachers. As of 2015, access requires a monthly fee.
Homeschoolmath.net features math videos, worksheets, articles, and curriculum guides arranged by grade level free of charge, focusing on mastery of concepts rather than memorization. Khan Academy, a non-profit site dedicated to spreading knowledge, contains hundreds of videos teaching a wide variety of subjects. While lessons are not specifically designated as being for third-grade students, they cover content from basic addition and subtraction up to calculus and are appropriate for users of all ages.