Physical activity is not only important for your child's health - it will help them cope with the sheer physicality of interacting with twenty children on the playground. Bumps and shoves are inevitable, so make sure your child has lots of physical play to develop gross motor skills too.
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.
These are basically digital versions (like ebooks) of worksheets that can be bought and downloaded to your computer from the internet or from CDs and then printed out. The advantage is that you can print out whichever sheet you wish. If you need to re-inforce a certain concept more you can usually print out that sheet again. Of course you will therefore need to have a computer and a printer.
When your child is ready, move on to pencil and paper. Build confidence by letting your preschooler trace simple lines and shapes, then proceed to letters. You can eventually teach your child to write his name by letting him trace or copy it daily. If your child needs help remembering how to spell her name, practice with fridge magnets, letter tiles or alphabet blocks.