This is one of those questions where a diagram comes in handy.

Since each face (technical name for a surface) is a rectangle then each side must be a factor of any 2 of 12, 15 and 20.

It follows that in this example your side lengths are 3, 4 and 5 since 3*4 = 12, 3*5 = 15 and 4*5 = 20 (this matches up to the diagram I drew)

Now you know the side lengths it is trivial to find the volume