a block of (1/3) x 1 x 1, has a volume of, just that product, namely 1/3.
a Unit Cube, has all sides of 1 unit, thus called the Unit cube, and has a volume of 1*1*1 = 1³ = 1.
so, how many times does the volume of 1/3 go into 1?
recall there are three thirds in a whole, so 1/3 + 1/3 + 1/3 = 1, 3 times.