<p>This research examines the impact of various core shapes and face sheet thickness on composite sandwich structures. This study aims to enhance the strength-to-weight ratio, also known as specific strength, a crucial parameter in the design and optimization of structures. The innovations of this work are the fabrication of a three-layer composite structure, with two composite layers of glass fiber-reinforced plastic (GFRP) by vacuum pump and a mesh core made of acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) materials by fused deposition modeling (FDM) (3D printer). The other innovations, including compression, three types of cores (tubular, honeycomb, and truss), are used to consider the effect of the geometric shape of the printed core, as well as two different thicknesses of the sheet layers on the honeycomb core. The samples are manufactured by ASTM standards, undergoing tensile, three-point bending, buckling, and vibration tests. To verify the experimental results, the behavior of the samples under bending, buckling, and vibration has been obtained numerically. The equations of motion based on Hamilton’s principle and the equations of equilibrium using the minimum potential energy method have derived based on the Euler–Bernoulli beam theory. Subsequently, the deformation due to bending, critical buckling load, and natural frequency of the three-layer composite sandwich structure has been calculated using the Navier’s method. Theoretical, experimental, and numerical (Abaqus) analyses validate the buckling and the vibration responses. Three-point bending tests further evaluate mechanical behavior. The results of this experiment demonstrate that the tubular core, due to its symmetrical geometric structure and absence of stress concentration points, performs more effectively against the applied loads induced by the test, including tensile, bending, and compressive loads. The critical buckling load and the ultimate failure force due to the bending load of the tubular core have increased by 1.84 and 1.98 times compared to the honeycomb core, respectively. Also, this core has enhanced the critical buckling load and the ultimate failure force by 1.15 and 1.14 times compared to the truss core, respectively. In addition, the honeycomb structure, because better stress distribution and displacement caused by bending loads within the cells, leads to increase the energy absorbed in the structure by 7% and 71.9% compared to tubular and truss cores, respectively. Among the innovations of this work are the construction and the investigation of the structural mass. Increasing the thickness of the composite layer in the honeycomb core while slightly increasing the structure’s mass has significantly improved its behavior. Notably, increasing face sheet thickness in honeycomb cores significantly enhances structural performance, improving braking force and specific energy absorption by 73% and 24%, respectively. In addition, one of the novelties of this work is the incorporation of a lightweight and compact core, which creates a structure that significantly reduces the final price while increasing bending stiffness and, consequently, the moment of inertia. Considering the unique characteristics of the structures, including high specific strength, they can be widely used in the wind turbine industry.</p>