Many online math games are designed purely for entertainment and won't do much in terms of teaching your child. While these games can be fun and engaging for third graders, you want to choose games that will practice the skills being taught in school.
Since 3rd grade math relies on the concepts that were learned during kindergarten, first and second grades, don't be afraid to start your child at a lower level.
Third grade can be particularly challenging when it comes to math, as this is the year that students are learning about fractions, measuring and weighing objects, graphing and counting money. Most importantly, third graders should be comfortable with the basics of math such as adding, subtracting, multiplying and dividing. If your child isn't comfortable with these basic components, it's almost inevitable that he or she will struggle with future math lessons.
Worksheets also, when used properly, provide both the students and parent / tutor immediate feedback as to the child's progress. This means they can be used to point out areas where the student needs further reinforcement.