The solution by @josgarithmetic, giving the answer 13, 15, 17, is INCORRECT.
The correct answer is -13, -11, -9.
Solution
Let the numbers be n, n+2, n+4.
Write an equation as you read the problem
{2(n+(n+2)) = 3(n+4)-21
Simplify and find n
2(2n+2) = 3n+12-21
4n + 4 = 3n - 9
4n - 3n = -9 - 4
n = -13.