# Plotting cuboid in a ellipsoid

I'm having trouble to implement a plot in Mathematica. I want to plot two things at the same time.

The first thing is a ellipsoid given by $\displaystyle f(x,y,z)=\frac{x^2}{a^2}+\frac{y^2}{b^2}+\frac{z^2}{c^2}=1$.

The code I wrote for this is:

a = 3; b = 2; c = 1;
ContourPlot3D[x^2/a^2 + y^2/b^2 + z^2/c^2 == 1, {x, -3, 3}, {y, -3, 3}, {z, -3, 3}]


The second thing I want to plot is the cuboid with the maximized volume: $\displaystyle f\left(\frac{a}{\sqrt{3}},\frac{b}{\sqrt{3}},\frac{c}{\sqrt{3}}\right)= \frac{8}{3\sqrt{3}}xyz$.

The problem is that I want the cuboid to be in the ellipsoid and I can't write anything that works.

Graphics3D[{