That depends on how many of the digits are repeated. If no digits are repeated, you have 9 choices for the first digit, 8 choices for the second, etc.; for a total of 9 x 8 x 7 x 6 x 5 x 4.
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There are 5,461,512 such combinations.
64
6 of them.
I am assuming you mean 3-number combinations rather than 3 digit combinations. Otherwise you have to treat 21 as a 2-digit number and equate it to 1-and-2. There are 21C3 combinations = 21*20*19/(3*2*1) = 7980 combinations.
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