When there are two units to be compared with each other, we can have only TWO degrees of comparison, i.e. positive and comparative.
e.g. John is not as tall as Akbar (positive). Akbar is taller than John (comparative). (No superlative degree)
The questions can be: a. Who is the taller of the two? b. Who is taller than the other?
When there are three or more units to be compared with one another, we can have all the three degrees, i.e. positive, comparative and superlative.
e.g. Akbar is taller than John (comparative). John is not as tall as Akbar (positive). Ashok is the tallest of the three (superlative).
The questions can be: a. Who is the tallest of the three? (Ashok) b. Who is taller than the other two? (Ashok) c. Who are not as tall as Ashok? (John and Akbar)