This study explored a new impact protection method based on three-phase locally resonant structures (LRSs), and this method did not rely on large structural deformation and material destruction. The performances of three LRSs with a steel cylindrical oscillator (LSCYO), aluminium cylindrical oscillator (LACYO), and steel cubic oscillator (LSCUO) were studied. A design with an optimised topology with the goal of maximising the impact attenuation was introduced to further improve the performance. The nonlinearities of the material, geometry, and contact were simultaneously considered in the optimisation of the design. The analysis results indicated that these optimal LRSs were anisotropic and could efficiently attenuate the impact load and absorb the impact energy. The attenuation effects of these optimal LRSs were better than those of the original and LRS models. In addition, the mechanisms of the energy absorption and conversion in the impact attenuation process were analysed. The vibration modes and propagation behaviours of the waves of these optimal LRSs were also studied. The attenuation frequencies of the waves and natural frequencies of these optimal LRSs were in good agreement, and the attenuation frequencies of the waves were all within the spectral range of the impact loads. In addition, multiple optimal LRSs were applied simultaneously to study their impact response, and the attenuation effect of the impact force was demonstrated. In short, this study investigated the attenuation mechanism of LRSs from the perspective of energy conversion, explored the new application scenarios of LRSs, and provided a novel solution for the impact problem, which is of significance in the field of functional structure design and impact protection.