This activity was specifically designed to meet the CCSS for first grade. There are 3 worksheets that can be used to practice adding 3 numbers in isolation with the support of number boxes. Then there are 3 worksheets with the same boxes that students can use to build number sentences and solve them.
**Please note that these templates are the same as the ones I used in my First Grade Operations & Algebraic thinking packet, but the numbers and word problems are DIFFERENT. This would complement that packet or work well by itself.**
CCSS.Math.Content.1.OA.A.2 Solve word problems that call for addition of three whole numbers whose sum is less than or equal to 20, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.