What is the mass of 3 liters of water?

At 4°C pure water has a density of 1000 kg/1 m3.

1 litre = 1000 cm3

3 litre = 3000 cm3 = 0.003 m3

Mass = Density x Volume

= 1000 kg/1 m3 x 0.003 m3 = 3 kg