You have three cups which differ in volume and shape. You know that the largest cup is less than three times the volume of the smallest cup. Each cup has a mark showing how much one-third is. You also have a drain.

The largest cup is already full of water. Pour water back and forth to fill all three cups to their one-third mark, pouring any excess down the drain.

Note: If a cup is less than one third filled, it may be filled up to the one third line BUT if a cup is over one third full it cannot be poured out to reduce it to exactly one third filled because when the cup is tipped, you have no indicator for one third full when tipped.