If repetition is allowed and leading zeros are allowed, then you have 000 to 999, which is 1,000 possibilities (I will not list them here). If no leading zeros are allowed, then you have 100 through 999, so there are 900 possibilities. If there are other restrictions, such as no repetition, then I'll refer you to the related link at MathsIsFun.com
There are 60480 numbers.
1 set
There are 5! = 120 such numbers.
There are 900 of them.
6*6*3 = 108 numbers.
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
27 three digit numbers from the digits 3, 5, 7 including repetitions.
There are 60480 numbers.
840
24
500
There are 3024 of them.
180 of them.
There are 5*4*3 = 60 such numbers.
1 set
It is 120 if the digits cannot be repeated.
There are 5! = 120 such numbers.