What is the matrix of inertia of a thin rectangular plate?

Question

I only want to get sure how the matrix looks like,

Is it : $\begin{bmatrix} \frac{Mb^2}{3} &0 &0 \\ 0&\frac{Ma^2}{3} & 0\\ 0& 0 & \frac{M(a^2+b^2)}{3} \end{bmatrix}$

Or not ?(a=length,b=width,M=mass)