In the context of aeronautics, automotive and construction applications the design of lightened multilayer plates with optimized vibroacoustical damping and isolation performances remains a major industrial challenge and a hot topic of research. This paper focuses on the vibrational behavior of three-layer composites plates in broad-band frequency range. Several aspects are studied through measurement techniques and analytical modeling of two sandwich systems: symmetrical steel/polymer/steel and nonsymmetrical steel/polymer/aluminum plates. Contactless measurements of the velocity field made using a scanning laser vibrometer allow estimating the equivalent single layer complex rigidity in the mid/high frequency ranges. Coupled with low/mid frequency estimations obtained with a high-resolution modal analysis method, the frequency dependent equivalent Young’s modulus and equivalent loss factors of the two composites plates are finally identified for the whole [0-5 kHz] band. The results are in very good agreement with an equivalent single layer analytical modeling based on wave propagation analysis. The comparison with this model allows identifying the frequency dependent complex modulus of the polymer core layer through inverse resolution. Dynamical mechanical analysis measurements are also performed on the polymer layer alone to compare with the values obtained through inverse method. Again, a good agreement between these two estimations over the broad-band frequency range demonstrates the validity of the approach.