3rd Graders Math Educational Resources for Kids Online - SplashLearn 3rd Graders Addition Educational Resources for Kids. Use your addition skills to add 3-digit and 1-digit numbers with regrouping. Regroup to add 3-digit and 1-digit numbers to play this game. Kids must add and subtract 3-digit and 2-digit numbers with regrouping to enhance their math skills. Solidify your math skills by adding and subtracting 3 ...