Lesson objective: Understand that we can add more than two numbers.
Students bring prior knowledge of solving addition and subtraction word problems within 10 (K.OA.A.2). This prior knowledge is extended to solving problems that involve three addends whose sum is less than or equal to 20. The concept is developed through work with linking cubes, number lines, counters, bar models and ten frames, which shows how numbers can be decomposed (broken apart) and composed (put together) with three addends. A bar model is used above a number line, connecting discrete and continuous representations. While students work with the concept of composing and decomposing values, they are not held accountable for using the words "compose" or "decompose" in first grade.
The understanding that we can add more than two addends lays the foundation for students to understand and apply the commutative and associative properties of addition. While students have encountered the commutative property previously in Unit 6, they encounter the associative property for the first time in Lesson 7 of this unit. Though students understand and use the properties, they are not held accountable for the terminology. Students will apply their understanding of addition and subtraction and begin to refine their addition strategies for subtraction within 100 in Grade 2, Unit 1.
This work helps students deepen their understanding of operations because students build on their understanding of addition as putting together by adding more than two addends.
A conceptual challenge students may encounter is thinking they can only add two numbers. Students may also require extra time to work with composing and decomposing numbers.
These concepts are developed as students have time to experience and explore composing and decomposing using tools such as ten frames, counters, bar models, and number lines.
Students engage in Mathematical Practice 7 (Look for and make use of structure) as they look for the structure in the numbers to find ways they can decompose them into three smaller numbers. For example, students may first decompose the number 17 into ten and seven and then further decompose the ten (drawing on their previous experiences of making ten) to create a problem with three addends.
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)

addition: a mathematical operation of combining two or more numbers into a total or sum

sum: the result when two or more quantities are added

total: the whole amount or value
Special materials needed:
 linking cubes or counters
 number lines (020)
 ten frames