Lesson objective: Add three addends by grouping known facts such as tens or doubles to help solve problems more efficiently.
This lesson helps to build fluency with addition of three addends. A ten frame is used here because it supports the concept of making a ten and adding a value to ten. This work develops students' understanding that when we are adding numbers, it doesn't matter how they are grouped.
Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they use facts they already know in order to make adding three numbers more efficient.
Key vocabulary:

addend: one of the numbers in an indicated sum of two or more numbers (e.g., 4 + 3 + 1 = 8; 4, 3, and 1 are addends)

double fact: an addition equation in which each addend is the same

equation: a mathematical sentence stating that two expressions are equal

group: a number of people or objects that are assembled together or that have some unifying relationship

known fact: an equation that can be automatically and correctly stated without counting

sum: the result when two or more quantities are added

ten fact: an addition equation with two addends that total or equal ten
Special materials needed:
 double ten frames
 multicolored counters