Teaching reading and other language arts skills to preschool learners relies heavily on a concept known as modeling. Modeling merely means exposing young children to the types of behaviors and habits we want them to learn. Many parents and homeschooling educators employ this kind of instruction naturally, without giving it much thought.
If your third grader needs help with math, there are many useful tools that can downloaded directly from the computer. Math is a subject that is best taught with visual aids, making the lessons more tangible for students.
This sounds like an awful lot, doesn't it! A good set of preschool worksheets should cover all of this, and more. In the preschool years, repetition is the key to learning, but you should look for worksheets that teach the same skills in a variety of ways. This not only prevents boredom setting in, but also reinforces the concepts by encouraging understanding as well. The importance of reading to your child cannot be emphasised enough, and you should encourage them to 'read' as much as possible 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.