There are an infinite number of 3 number sets that will equal 9000, unless your looking for whole numbers.
If you want to find a set of whole numbers, first break 9000 into its factors:
5 * 5 * 5 * 3 * 3 * 2 * 2 * 2 = 9000
Now to find a set of 3 numbers, just combine some of the factors.
For example:
(5 * 5 * 5) * (3 * 3) * (2 * 2 * 2) = 9000
125 * 9 * 8 = 9000