Let "x" will be an unknown middle number. Then the three numbers are (x-1), x and (x+1). You are given that (x-1) + x + (x+1) = 81, or, which is the same, 3x = 81. Hence, x == 27. So, your numbers are 26, 27 and 28.
If you are given the sum of three consecutive integer numbers, the middle number is ONE THIRD of the sum. It is ALWAYS TRUE and may help you to solve similar problems MENTALLY.