Here is how guest got that answer :
If all the digits are differerent we have C(10,3) = 120 sets
If two digits are the same and the other is different, we hve 10 ways to pick the two digits and 9 ways to select the remaining one = 10 * 9 = 90 sets
If all 3 digits are the same, we have 10 sets
So
120 + 90 + 10 = 220 sets (ways)
