In this paper we present our proposal of processing three-beam interferograms obtained in a Fizeau interferometer when testing quasi-parallel optical plates. If the intensities of three interfering beams reflected from the front and back surfaces of the investigated plates and from the reference flat are nearly equal, the modulation distribution of the pattern encodes the information of the plate optical thickness variations, whereas the phase distribution contains the information about the sum of profiles of both surfaces (uniform refractive index distribution is assumed). Both maps can be derived using a combination of different interferogram analysis techniques such as Temporal or Spatial Carrier Phase Shifting and Vortex Transform. As a result separate information about both surfaces from a single measurement can be obtained. The situation complicates, however, when noticeable difference of the beam intensities occurs. We prove that in this case the modulation still contain useful information on the optical thickness of the investigated plate. Numerical studies of the method working principle are complemented by experimental results.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.