Add three 3-digit numbers

Each column may now sum to a larger value, making double-digit column sums (and thus carrying 2 instead of 1) more likely — students must be prepared to handle carries greater than 1.

Adding two of the three numbers first, then adding the third to that partial sum, breaks the problem into two familiar steps and reduces errors compared to adding all three at once.

They directly address 3.NBT.2, which requires students to fluently add within 1,000, and prepare students for the multi-addend addition common in 3rd grade word problems.