Students can practice third-grade math online at free education sites such as Khan Academy, Adapted Mind, Math ABC and IXL. All of these sites break down math topics by grade and offer many free practice sheets and tests.
A student or parent can create a free account at any of these sites, but the sites also allow users to practice without signing up. Choose the grade and mathematical subject, and start practicing. A few of the sites offer how-to videos for different concepts. The sites are primarily designed for practicing concepts learned in the classroom, but they are not classroom substitutes.
Most of the sites gear toward children, with pictures or animations of cartoon characters that assist and encourage the student while practicing. For a student with a user account, these sites also track the student's progress and dole out virtual badges for various achievements, such as for a certain number of questions completed or correct answers given.
Math curricula may vary between states, and it is worthwhile to ask a student's teacher if a full math syllabus for the year is available or whether the teacher has any recommendations for practice at home, using an online program or otherwise.