Why are complexes of type MA3B3 not optical active?
Orient the fac- isomer so one A atom is facing towards you and a space between two B atoms is below it.  Then the left half of this view is a reflection of the right half so you are actually looking down a mirror plane.
Because you could have chosen any of three A atoms in your view there are three mirror planes, which pass through a common rotation axis.  The axis passes through the face that has all A atoms at its corners and the opposite face which has all B atoms.  This symmetry is called $C_{\mathrm{3v}}$ in the language of point groups.