Sandwich structures are widely used in lightweight engineering applications due to their high stiffness-to-weight ratio and structural efficiency. With the advancement of additive manufacturing (AM), the integration of complex lattice architectures as cores in sandwich beams has become increasingly viable, enabling the fabrication of highly optimized structures with fewer manufacturing steps and improved performance [1]. The present study investigates the mechanical response of sandwich beams with additively manufactured lattice cores and composite facesheets under three-point bending. The structures under consideration consist of cross-ply CFRP facesheets and an additively manufactured AlSi10Mg lattice core of f2ccz topology. An analytical model based on the First-Order Shear Deformation Theory (FSDT) has been developed to predict deflection and stress resultants [2]. In addition, finite-element (FE) simulations (ANSYS) have been performed for both non-homogenized (explicit lattice) and homogenized (orthotropic-equivalent) cores. The present study investigates the impact of core aspect ratio and facesheet-core thickness ratio on deflection, normal stress, and transverse shear. The results show strong agreement between FSDT and FE in terms of global bending and deflection responses, confirming the applicability of FSDT for lightweight AM lattice-core beams. The homogenized core effectively reproduces the overall response with reduced computational cost, while the explicit lattice model captures local stress concentrations near strut intersections. These findings emphasize the importance of core shear in compliant cores and provide guidance on when homogenized modeling and FSDT are sufficient for structural prediction and when detailed lattice-resolved FE analysis is required.