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.
Kindergarten worksheets are often of much value in helping kindergarten children learn and re-inforce basic concepts in an interesting way. Small children usually like to do things rather than just read or listen. They also get bored rather easily. Therefore giving them well-designed, illustrated worksheets to do makes it easier and more fun for them to learn. Completing a worksheet also gives a child a great sense of fulfillment. In fact, not just for kindergarten, but even for older learners worksheets can form a valuable part of the learning process.
When you feel your child is physically ready to write, have your child use a stick or finger to draw in sand, rice, pudding, shaving cream, paint or oatmeal. Make simple lines and shapes and ask your child to copy them.
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.