Take the three numbers, and list all of their prime factors. I'll use 3, 9, and 21 for an example to help.
3: 3
9: 3 x 3
21: 7 x 3
You take the times that a number occurs the most in any of the three numbers and multiply them together. 3 occurs the most in a single number twice, and 7 occurs the most in a number once.
So you'd multiply 3 x 3 x 7 to get 63.