amount: how much there is of something that you can quantify
quantity: something that can be expressed by a number (either constant or variable.
volume: 3*3.785 litres (11.355 litres)
amount: What is the amount? Little, much, right amount, regular mount, certain amount -- how much of something is available -- use an indefinite quantifier.
quantity: (the number of gallons) 3; 11.355 kg's of water, 11.355 litres of water