Each cube here is 1 inch on its edge. the volume is square inches.

The dimensions of the rectangular prism formed by the small cubes are
 
5 by 4 by 3 inches, where 3in is the height, 4in is the width and 5in is the length, as we can see from the figure.

Then the Volume is = height*width*length= 3in*4in*5in=60 in  cubed.

OR, 

the volume of each small cube is 1*1*1=1 in cubed.

on the top layer, we can count 5 rows of 4 cubes each row, which makes 20 cubes in the top layer.

there are 3 layer, so in total there are 3*20 cubes per layer = 60 cubes.

Thus, the total volume is 60 * 1 in cubed, per cube = 60 in cubed

60 in cubed