Mechanical metastructures, owing to their exceptional mechanical performance and structural adaptability, have shown broad application potential in fields such as aerospace and advanced engineering. However, most existing metastructures are fabricated using additive manufacturing (AM), which often results in fixed mechanical properties and limited tunability. To overcome these limitations, this study proposes a programmable discrete assembly approach based on L-shaped modular elements. The proposed module exhibits geometric compatibility, allowing the construction and topological transformation of three representative lattice architectures—Octet, FCC, and Cuboctahedra—through variations in spatial configuration, thereby overcoming the structural singularity inherent in conventional assembly methods. A hybrid fabrication process combining 3D printing and mechanical fastening was adopted, achieving support-free printing while maintaining high fabrication efficiency and cost-effectiveness for metastructures. Finite element simulations were employed to systematically investigate the mechanical responses of the three discretely assembled lattices, elucidating the intrinsic relationships between lattice topology, stiffness, strength, and energy absorption characteristics. Furthermore, two performance modulation strategies—soft-hard layered hybridization and local lattice hardening—were proposed to enable programmable control of global and local mechanical properties. This study establishes a new design framework for tunable mechanical metastructures, providing an effective pathway for customized performance and lightweight design in large-scale aerospace and multifunctional structural applications.