I'm not sure I understand what you're looking for.
SELECT DISTINCT a, b, c
is going to return a recordset that has one row for each distinct
combination of a, b and c. For any given a and b combination, you'll only
get (at most) 1 instance of each possible c value. You will see specific
values of c multiple times, but only if a and b are different.
Can you give an example of exactly what you're looking for? If you only want
to see a specific value of c once, how do you want to choose which of the
multiple a and b values to display?