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.
If your child is used to doing a worksheet or two, they will be able to do this quite easily, and will understand that they need to complete the task before they can move on to the next activity. Worksheets also help your child to learn how to follow instructions, and teach them about following rules.
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.
Make sure your child is not tired or hungry so that he or she can focus all attention on learning. Also try to keep the lessons consistent with what is being learned in school. A quick chat with the teacher or signing up for an online newsletter from the classroom are ways to keep tabs on the lesson plans.