One good math question to ask a third grader is, "If you divide 13 apples into two equal groups, how many apples are leftover at the end?" This helps them to learn about remainders in long division.
It is also appropriate to ask third graders questions about fractions, such as "What is 3/9 + 1/9?" Third graders are also learning to multiply, so quizzing them on their basic multiplication tables is useful, with questions like "What is 3 x 9?" or "What is 12 x 4?" Questions involving subtracting with regrouping and zeros are also a challenge, such as "What is 985 - 694?"