6
32C3 = 4960
45
Six combinations: 123, 132, 213, 231, 312, 321
Assuming you are treating each number as a number and not as an individual unit, the numbers you can make from these digits are 899, 989 and 998.
017, 071, 107, 170, 701, 710. 6 combinations
10
9*9*9 = 729 using the digits 1 to 9 and 2*9 using 10 and another digit. 749 in all.
We notice that you said "combinations of three digits". You didn't say "3-digit numbers".There are (10 x 10 x 10) = 1000 combinations of three digits.The only combinations of 3 digits that form numbers less than 15 are 000, 001, 002, . . .012, 013, and 014. There are 15 of those.So the remaining (1000 - 15) = 985 are all larger than 15.
-8
There are 720 combinations if you use each of the digits only once per combination.
i guess the answer is 54. A group of three digits can be selected from the 5 digits in 9 ways and each group can be arranged in 3! (i.e., 6) ways respectively and hence the answer is 6*9=54