The least of these numbers is 21.
Since the numbers are consecutive, let 3x, 3x + 3, and 3x + 6 represent these numbers. Add these values, combine like terms, and solve for x.
3x + (3x + 3) + (3x + 6) = 72
9x = 63
x = 7
The smallest number is 3(7) = 21