Additive manufacturing (also known as 3D printing) technologies are successfully used in various applications as they offer many advantages in production, such as (i) less material consumption and (ii) shorter manufacturing times, factors that reduce costs. In recent years, experts in 3D printing have focused their studies on designing and printing cellular structures. This structure's advantages (high strength-to-weight ratio, high flexural stiffness-to-weight ratio, high-energy absorption capacity, thermal and acoustical insulation properties) make it widely used in aerospace, sustainable energy, marine, and automotive industries. This study aims to study the mechanical properties of sandwich structures manufactured using polylactic acid (PLA) material with rhombus and honeycomb core shapes as a single part by an FDM 3D printer. First, the functional properties of the sandwich structures were quantified by shape evaluations. Then, tensile, three-point bending, and compression tests were performed to determine the mechanical performance of the different samples. The results show that rhombus structures gave better mechanical behaviour as the tensile, bending, and compression strengths were 15.3%, 39.8%, and 35.1%, respectively, higher than the honeycomb, indicating their reliable core construction.