<p>Although the concept of metastructures is widely applied for vibration attenuation, the influence of the complexity of the geometry of three-dimensional (3D) panel elements remains a significant gap in the literature. The novelty of this work lies in demonstrating passive control of waves and vibrations in 3D metapanels at low frequencies solely through geometric and material configuration. In this context, this study investigated the formation of full band gaps in metapanels with a square lattice core in vacuum, considering variations in geometry and polymeric materials. The investigation was conducted using COMSOL Multiphysics and the finite element method (FEM). The results revealed that vacuum and materials strongly influence the formation of full band gaps, particularly in complex 3D configurations. The presence of vacuum heightened the system’s sensitivity to the thicknesses of the core and plates. The band structure analysis demonstrated that full band gaps occur only when the core and plates have equal thicknesses, with the phenomenon disappearing beyond critical limits. The combination of denser polymers in the core with less dense polymers in the plates increased bandwidth, while the use of three materials only offered significant advantages in some specific cases. The choice of materials and geometric thickness was crucial for optimizing the dynamic properties of metapanels, but improper configurations could entirely hinder the formation of full band gaps. The study numerically demonstrates that metapanels can exhibit full band gaps associated with the periodicity and local resonance of thin plates, without the addition of inclusions or resonant elements. This phenomenon enables metapanels, when designed with appropriate geometric and material variations, to passively control waves and vibrations within specific frequency ranges.</p>