The paper presents an original physical points of an intellectual CAD system designing for three-tier wide band waveguide-dielectric filters with LM modes. On the basis of formalized physical knowledge about the behavior of coupled resonators, the CAD system analyses electromagnetic signal passing through a filter structure and makes decisions gradually approaching the optimal filter design through a series of changes in its geometry. From a mathematical point of view, this approach is an alternative to well-known optimization methods and is also very promising for solving the problem of finding a global extremum of an objective function. We also include the filter optimization results obtained with the CAD system for the frequency range 23 - 81 GHz, band widths up to 30% and insertion losses less than 0.5 dB