We present the analysis of guiding properties of the 3D three-layer waveguides, taking into account the material's attenuators. The examined waveguide consists of dielectric rod having rectangular cross section, and thin film of greater refractive index than that of a rod, the given film is loaded on the rod. Attention is given mainly to the influence of the surfacing film's parameters on the phase and attenuation of the lightguide's guided modes. Modeling is based on the use of effective refractive index method in complex form. The film's normalized thickness dependences of phase and attenuation constants are computed for multimode waveguide.