If you include leading zeros in numbers like 001, or 032, and if you include the number 000, then there are one thousand three-digit numbers. If you do not include the leading zeros, then there are nine hundred such numbers. They go from 100 to 999 inclusive.
180 of them.
500
There are 34 = 81 numbers.
27
pizza
There are 7,290 different 4-digit numbers that can be formed from the digits 1-9 without repetition.
There are 3024 of them.
180 of them.
There are 60480 numbers.
840
24
500
27 three digit numbers from the digits 3, 5, 7 including repetitions.
There are 5*4*3 = 60 such numbers.
1 set
There are 5! = 120 such numbers.
The number 1234567890 can create about 39 of 40 4 digit numbers.