3 liters = 12 inches
volume unit = length unit?
??? Liters is a measure of volume, inches a linear measure, in this case depth. 12 inches would be just over 30 centimeters or 0.3 meters if one wishes to "go metric". In the volume has measurement of linear measurements cubed. For example cubic centimeters, with a special name, liter, for 1000 cubic centimeters. Another way of saying this that a milliliter (ml) is the same as 1 cc.
And where, pray tell, did you learn THAT little factoid?
liters = volume, inches = length. Not the same thing at all.