Lesson objective: Understand that we can group addends by known facts to solve problems more efficiently.
Students bring prior knowledge of making a ten from K.OA.A.4. This prior knowledge is extended to adding three numbers as students group them by known facts such as making a ten or doubles. A conceptual challenge students may encounter is thinking they must add the numbers in order or start with the larger number even though there may be a more efficient way.
The concept is developed through work with a double ten frame, which helps build the concept of making a ten.
This work helps students deepen their understanding of equivalence because any way we group the numbers, they have the same sum.
Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they determine the meaning and work towards finding a possible solutions. Students will also engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they look for ways to decompose numbers into three addends that include partners of ten.
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

decompose: to break a number into smaller units to simplify computation
 double fact: an addition equation in which each addend is the same

equal: having the same amount, size, number, or value

equation: a mathematical sentence stating that two expressions are equal

sum: the result when two or more quantities are added
 ten fact: an addition equation including two addends that total or equal ten
 ten frame: a visual array model that has two sets of five connected squares
Special materials needed:
 double ten frame
 multicolored counters