The first digit can be any one of the 9. For each of these . . .
The second digit can be any one of the 8 not already used. For each of these . . .
The third digit can be any one of the 7 not already used.
Total number of possibilities = (9 x 8 x 7) = 504
500
180 of them.
It is 120 if the digits cannot be repeated.
There are 34 = 81 numbers.
I take it that you want to make three digits numbers with 8,7,3, and 6 without repetition. The first digit cane be selected from among 4 digits, the second from 3 digits, the third digit from 2, hence the number of three digit numbers that can be formed without repetition is 4 x 3 x 2 = 24
There are 7,290 different 4-digit numbers that can be formed from the digits 1-9 without repetition.
10,000
36
27 three digit numbers from the digits 3, 5, 7 including repetitions.
24
500
There are 60480 numbers.
840
If the digits can be used more than once, then 900. If not, then 648.
There are 3024 of them.
180 of them.
There are 5*4*3 = 60 such numbers.