if the jar top to bottom same size then the answer is

first fill the water in three litre  jar , and put that water in five litre jar and now   mark the water level , after measuring fill the water full in that five litre jar, after that  the empty three litre jar fill the water since the measurement reach in five litre jar  , so now the water present in the jar 2 liters
Two times  measure like that got 4 litres