We have to form all possible 3 digit numbers with distinct digits.we cannot have 0 at the hundred's place. so, the hundred's place can be filled with any of the 9 digits 1, 2, 3, 4 .... , 9.
so, there are 9 ways of filling the hundred's place.
Now, 9 digits are left including 0, so, ten's place can be filled with any of the remaining 9 digits in 9 ways. now, the unit's place can be filled which in any of the remaining 8 digits. so, th ere are 8 ways of filling the unit's place.
Hence, the total number of required numbers = 9 × 9 × 8 = 648