Adding two three digit numbers (for example 346 + 479) involves several steps.
Place one number above the other so that the hundreds, tens and ones places are lined up. Draw a line under the bottom number.
Add the ones place digits (9 + 6 = 15). This number is larger than 10 so place a one above the tens place column and place the five below the line in the ones place column.
Add the tens place digits (1 + 4 + 7 = 12). This number is larger than 10 so place a one above the hundreds place column and place the two below the line in the ones place column.
Add the numbers in the hundreds place column (1 + 3 + 4 = 8) and place the 8 below the line.
Learn to explore 3 – digit addition with and without regrouping the tens below.