3! = 1 x 2 x 3 = 6. The same for any three different digits - you have 6 possible combinations.
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NO! That is the number of permutations! In a combination the order of the numbers does not matter, so there is only one combination 123. And that is the same as 132, or 312 or ...
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123
There are 3 values (1, 2, 3) for each of the 4 digits. Therefore, there are 3*3*3*3 or 81 four digit numbers that can be formed.
Just think of how many possibilities you have for each digit. 10x9x8x7x6x5x4x3x2 or 10 factorial is 3 628 800.
You can make six different combinations: 123 132 213 231 321 312. If you just need to know the number of combinations, you could find the answer without writing everything out by letting x represent the number of digits and calculating x*(x-1).
If you can repeat a digit, there are 27. If you can't repeat a digit, there are only 6.