There are 3 ways to choose the first digit, either 1, 2 or 3.
For each of those 3 ways, regardless of which one you chose for
the 1st digit, there remain 2 digits to choose for the second digit.
So there are 3×2 or 6 ways to choose the first two digits.
Then there is only 1 digit left to use for the 3rd digit.
So there are 3×2×1 or 6 ways to choose the three digits. Here are all
6 ways:
123, 132, 213, 231, 312, 321
Edwin