In forming a 3 digit number the 100’s place can be occupied by any 9 out of 10 digits(0 not included because it will lead to formation of 2 digit number.)

The 10’s place can be occupied by any of the remaining 9 digits(here 0 can or cannot be used.)

In one’s place any of the remaing 8 digits can be used.

So total 3-digit numbers with no digit repeated are:9 × 9 × 8=648.