Let n = your first number.
If they are consecutive numbers, then n + 1 = your second number and n + 2 = your third number.
It says "sum so we know to add them all together and solve for n. Solving for n then gives you the rest of your numbers.
n + n + 1 + n + 2 = 81
3n + 3 = 81
3n = 81 - 3
3n = 78
n = 78 / 3
n = 26
n = 26
n + 1 = 27
n + 2 = 28
Your numbers are 26, 27, and 28.