Six combinations if you use each digit only once per combination: 157 175 517 571 715 and 751.
1
6*7*7 = 294
A lot
There are only 999 three digit whole numbers.
6 ways: 931,913,139,193,391,319
There are no three didgit numbers but there are 63 three digit numbers.
999
171
There are 900 three-digit numbers.
300300300300
10
45