Addition of Three-Digit Numbers Math Worksheets | Aged 6-8 In addition of 3 digits, place the first addends above the second addends so that the ones, tens, and hundreds line up. Add the numbers in ones place, then add the tens place, and lastly add the numbers in the hundreds place. If the sum is over 9, you carry over the 1 to the next place We use addition of 3-digit numbers in our daily