Answer:
64
Explanation:
Consider the possible 3 digit numbers as _ _ _
where each blank represents a digit
The leftmost blank can be filled using one of the 4 numbers
The middle blank can also be filled in 4 ways using one of the 4 numbers
The rightmost blank can also be filled 4 ways
Total number of 3 digit numbers = 4 x 4 x 4 = 64
Additional Note
If repetition is not allowed, first space can be 4 ways, second 3 ways and third 2 ways giving: 4 x 3 x 2 = 24