Let x y z be the number.
Any one of the nine digits from 1 to 9 can be the value of 'x' . But for 'y' , zero can also be a choice. Hence if there was no restriction on the number, 'y' could have assumed 10 different numbers. Now we know that, whatever value was chosen for 'x', the same value cannot be chosen for 'y'. hence only 9 possibilities remain for 'y'. Similarly, 8 different possibilities are there for 'z'.

 

Hence there  9 x 9 x 8 = 648 such numbers.