A mathematician can...
(3.NBT.A.2) Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
(3.oA.D.8) Solve two-step word problems using addition and subtraction. Represent these problems using equations with a letter standing for the unknown quantity. Assess the reasonableness of answers using mental computation and estimation strategies including rounding.
Below are some websites to review and practice the standard.
Website Standard http://www.mathplayground.com/grade_3_games.html Practicing addition and subtraction
http://www.mathplayground.com/mathvideos.html Practicing addition and subtraction strategies
http://www.xpmath.com/ Practicing addition and subtraction
www.fun4thebrain.com/mult.html Multiplication and division fluency practice
http://www.mathplayground.com/mathvideos.html Practicing addition and subtraction strategies
http://www.xpmath.com/ Practicing addition and subtraction
www.fun4thebrain.com/mult.html Multiplication and division fluency practice