3 -digit numbers have to be formed by using the digits 1 to 9 .Here, the order of the digits matters.Therefore, there will be as many 3 -digit numbers as there are permutations of 9 different digits taken 3 at a time.Hence, required number of 3 -digit numbers are9P3​=(9−3)!9!​=6!9!​=6!9×8×7×6!​=504 .