Look for games and puzzles that are part of an adaptive learning program. This means that the online games are well-thought out and match the same set of skills that are being taught in the third grade curriculum. The program is structured toward each individual student and fills in the gaps where the child is struggling.
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.
A comprehensive set of worksheets covering a variety of subjects can be used to expand your child's learning experience. A worksheet about shapes can be used as part of a game to find shapes around the house, counting worksheets can be used to count things you see in the grocery store and so on.
Quality worksheets for preschool can help you with a lot more than just academics. For example, once your child starts kindergarten, they will be expected to sit still and complete tasks for about 15 to 20 minutes at a time.