Assuming those digits cannot repeat, there are different ways to arrange those digits. Hence there are 6 different numbers that can be made.
Another way of looking at it is that say I have those digits 1, 3, and 7.
Then I could place them in three spots _ _ _.
Choose a digit, lets say the 1. I can place that 1 in any of those three spots, e.g. _ _ 1. So far we have .
Now choose another digit, lets say the 7. There are only two spots where the 7 could be placed, so we have . Lets say we place it like so, 7 _ 1.
Now there is only one spot where the digit 3 can go so we have . Thus there are 6 different possible numbers.