Solve addition word problems involving three addends and a sum within 20. Example:

Chloe was at a petting zoo. She pet 2 lambs, 3 goats, and 1 pig. How many animals did Chloe pet?

Adding 5 + 3 + 6 by Khan Academy

Add 3 numbers by Khan Academy

Next, give your students this challenge:

1, 2, 3 Magic Square by NRICH

Conclude by leading this investigation:

Bubbling Cauldrons (single digit addition)

by MathPickle

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.