Every number consists of digits which have their own value. If the number consists of 3 digits, then the rightmost digit is for ones, middle digit is for tens and leftmost digit is for hundreds (it goes on and on with more-than-3-digit-numbers). About your task:
Hundred digit is smaller than tens digit and tens digit is smaller than ones digit. It means that the leftmost digit is the lowest, the rightmost digit is the greatest and the middle digit is a value between both of them.
For example:
123, 258, 346, 789, 278, 568 etc. etc.