how many positive three-digit numbers are there that have no digits repeating?
For the first digit, there are 9 choices (0 is excluded for a 3 digit number).
For the second digit, we exclude the first digit, but include 0, so there are still 9 choices.
For the third digit, we exclude the first two digits, so there are only 8 choices.
The rule of multiplication of the number of choices will give the final answer.