Solving 3-digit addition in your head
cc-early-math-add-sub-1000--cc-early-math-strategies-for-adding-two-and-three-digit-numbers--rethinking-three-digit-addition-for-mental-calculation
Addition and subtraction within 1000
Strategies for adding 2- and 3-digit numbers
Sal shows some techniques for doing mental addition, such as making groups of 10 and 100.