IF the three numbers are between 0 and 9 ( not picking three numbers out of a pool of larger numbers (45, 109, 86) the following would give you the winning number every time.

If what you want are all possible three digit numbers then you have 10 choices for the first digit, you have 10 choices for the 2nd digit, and you have 10 choices for the 3rd digit giving you 10x10x10 = 1000 in all.

If what you want are all possible three digit numbers with no repetition of the digits then you have 10 choices for the first digit, you have 9 choices for the 2nd digit, and you have 8 choices for the 3rd digit giving you 10x9x8 = 720 in all.