the volume would be V = (side of base)^2 * (height). The height would be 3. To see this, make yourself a model if need be. Assuming that the dimensions of the "square piece of cardboard" are x by x, then the length (or the width) of a side of the square bottom would be x-3-3, or x - 6.
So: V = (x-6)^2*3 = (area of base)*(depth of box)
= 3(x-6)^2 (answer)