That's exactly all the numbers you need to count from 000,000,000 to 999,999,999.
There are one billion of them.
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Exactly 3,628,800, or 10!.
10!/3! = 604800 different combinations.
Through the magic of perms and coms the answer is 729
If there are no restrictions on duplicated numbers or other patterns of numbers then there are 10 ways of selecting the first digit and also 10 ways of selecting the second digit. The number of combinations is therefore 10 x 10 = 100.
9!/6!, if the six different orders of any 3 digits are considered distinct combinations.