Only 6 entries will be unique, inertia matrices are always symmetric (i.e., Ixy == Iyx, Ixz == Izx, Iyz == Izy).  If you create a 3x3 with 9 unique entries and somehow you accidentally make it non-symmetric, it won't be representative of an inertia matrix and is probably a mistake.  The inertia() function in mechanics may be useful here.