How we can get pyramid capacity?

Question

lets talk width=3,length=2,height=1.5

Answer 1

It's 1/3 of the enclosing rectangular volume, or 1/3 of width x length x height, or, with your numbers: 1/3 * 3 * 2 * 1.5 = 3

It's hard to envision 3 of those pyramids filling up the enclosing rectangular volume.  It's should be easier seeing a cube cut diagonally to make six pyramids each is enclosed by a rectangular volume of 1/2 the volume cube.