Using infrared spectroscopy, we investigate bottom gated ABA-stacked trilayer graphene subject to an additional environment-induced p-type doping. We find that the Slonczewski-Weiss-McClure tight-binding model and the Kubo formula reproduce the gate
voltage-modulated reflectivity spectra very accurately. This allows us to determine the charge densities and the potentials of the {pi}-band electrons on all graphene layers separately and to extract the interlayer permittivity due to higher energy bands.
