Answer:
100
Step-by-step explanation:
We assume that you are concerned with integers, and that you only count a number as "3-digit" if it does not have a zero in the hundreds place.
In order for the number to have a minimum value, the minimum available digit must be used in each number place, starting from the most-significant place--the hundreds place.
The least (non-zero digit) available for the hundreds place is 1.
The least digit available for the tens place is 0.
The least digit available for the ones place is also 0, since it is allowed to duplicate a digit already used.
The least possible 3-digit number using 2 different digits is 100.
