This study investigates the influence of layer thickness and builds orientation on the surface hardness of Fused Deposition Modelling (FDM) 3D-printed thermoplastics, specifically polylactic acid (PLA), polyethylene terephthalate glycol (PETG), and polycarbonate (PC). Hardness testing was conducted using a Shore D durometer. Three-layer thicknesses (0.1, 0.2, and 0.3 mm) and three build orientations (flatwise, edgewise, and upright) were systematically evaluated to assess their effect on the hardness of the printed surfaces. Results showed a material-dependent response to layer thickness. PLA exhibited an optimal hardness at 0.2 mm, while PC's hardness decreased with thicker layers, and PETG remained relatively stable. Build orientation also influenced measured hardness, with flatwise specimens showing higher and more stable values compared to edgewise and upright builds, which exhibited greater variability due to interlayer exposure and reduced shell continuity. Infill ratio and pattern were confirmed to have negligible impact on hardness, highlighting the dominance of surface-related parameters in determining hardness response. These findings emphasize the importance of printing parameters in tailoring surface properties of FDM-printed components and suggest that hardness may be used as a complementary measure alongside other mechanical tests for quality control and application-specific optimization.