A capacitance of the third capacitor is equal to series connection of two capacitors formed by the gaps between the Cu slab and the plates.
The thickness of the Cu plate does not contribute to the capacitance.
I'm not sure what you intend with discussing the copper plate as dielectric. Its permittivity is almost purely imaginary, respectively it's not actually suited to make a capacitor. The permittivity parameter can be nevertheless useful to derive certain physical effects.