How much air does a 3 ltr dive cylinder hold?

A 3-liter dive cylinder holds approximately 3 liters of air at the cylinder's rated pressure. The actual amount of breathable air will depend on the pressure in the cylinder; for example, at a common pressure of 200 bar, a 3-liter cylinder contains about 600 liters of air at sea level conditions. This is because gases expand to fill the volume of their container, and the compression increases the density of the air inside the cylinder.