How many3 digit numerical passwords are there if a digit can be repeated

The first digit can be any one of 10 (0 - 9).  For each of those . . .
The second digit can be any one of 10 (0 - 9).  For each of those . . .
The third digit can be any one of 10 (0 - 9).

So the total number of possibilities is  (10 x 10 x 10) = 1,000 .

Another way to look at it:

If each digit in the password can be any digit, then the total number
of them is exactly how many counts you have when you count from
zero to the highest 3-digit number. 

Counting from  000  to  999, you have 1,000 counts.