The answer to the clue “Betaal aan het water” (Pay at the water) with the constraint of three letters is:
DOK
This word translates to “dock” in English and refers to a place where ships can be moored for loading and unloading cargo or passengers. In the context of the clue, “pay at the dock” could refer to various fees associated with using the dock facilities, such as mooring fees, harbor fees, or customs duties.