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.
Textbooks, instruction manuals, and lesson plans are the basic foundation for any homeschooling program. Homeschooling does not have to be limited to the parents understanding of the material that the student is learning. Home school information and education should instead be based on the student's individual learning style and the materials that must be taught based on the students grade level and educational needs.
Just because your child will be playing fun online games doesn't mean the same value won't be there. Create a comfortable study zone. Turn off outside distractions such as cell phones, radios or TVs.
There are many good workbooks available for kindergarten children. However with a regular workbook, a child can do each activity or worksheet only once. A good option is to use printable kindergarten worksheets.