Perpendicular axes theorem is applicable of two dimensional bodies or planer bodies having almost negligible thickness.

From figure the moment of inertia about rotational axis ($\mathrm{Z}$-axis) which is normal to plane of body is given by

$I_{\mathrm{Z}}=I_{\mathrm{X}}+I_{\mathrm{Y}}$ 

Where $I_x,I_y$ are moment of inertia of body about two mutually perpendicular axes in the plane of body.

Above theorem is not applicable for three-dimensional bodies.