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.
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.
With adaptive learning, the programs won't move on to the next level until your child has a firm grasp on the current material. The online games will be a wonderful way for your child to catch up on basic arithmetic concepts and be comfortable using them across applications.
Reading instruction is a component of the language arts curriculum that should be introduced across multiple subject matter content. Children ultimately have to understand what they read in a variety of textbooks, not just as part of a language arts class. Yet, reading instruction begins long before formal education begins, especially for homeschooling parents and educators.