Remove Duplicates From Bound Combobox

I am using a third party combobox (telerik) but it functions very similair to a regular asp.net combobox.

My problem is I have the combobox bound to a datasource, but I cannot do a DISTINCT query because not all rows are unique.

example of my sql table:

Col_1     Col_2      Col_3      Col_4
  hi           hey       yo           hey
  yo          hi          yo           hi

Lets say the combobox is bound to Col_3, Distinct wont work. and it will display two entries of "Yo" in the combobox.

any idea's on how to only display unique items "Col_3".

In my real table I have about 20 columns and each 20 combobox's is bound to a different