As you cannot repeat the numbers it is a permutation (not a combination) for the sake of calculating the number of possible outcomes.
The total number is 10! / (10 - 3)!, in English Ten Factorial divided by the Factorial of ten (number of available digits) minus three (number of times the digit is used).
or 10! / 7! = 3,628,800 / 5,040 = 720