This study presents a detailed numerical analysis of a three-dimensional micro pin fin heat sink incorporating 55 fins arranged in a single channel in four distinct cross-sectional geometries: square, circular, triangular, and pentagonal. A conventional microchannel heat sink (MCHS) without pin fins serves as a baseline for comparison. Water is employed as the working fluid, and simulations are conducted over a laminar flow regime with Reynolds numbers ranging from 500 to 1500. To efficiently capture the thermo-hydrodynamic behavior and reduce computational cost, a representative single flow channel is simulated under symmetrical boundary conditions, and key pin fin parameters such as height and spacing are systematically varied. The cross-sectional hydraulic diameter and spacing are held constant for all cases, with step sizes and non-dimensional spacing ratios sp/hp adjusted to assess their effect on heat sink performance. Results indicate that among all geometries, circular fins exhibit the highest heat transfer enhancement, with the Nusselt number increasing by 60% at Re = 500 and by 90% at Re = 1500 compared to the baseline. However, this improved thermal performance is accompanied by a greater pressure drop relative to the other tested pin fin shapes. Following the circular fins, triangular and square configurations offer progressively lower heat transfer rates, while pentagonal pin fins demonstrate the minimum enhancement. Furthermore, for all fin geometries, increasing the Reynolds number leads to a consistent improvement in heat transfer. Overall, the study provides quantitative insights into the impact of pin fin geometry and arrangement on the thermal and fluid dynamic performance of micro pin fin heat sinks, offering valuable guidelines for the design of advanced cooling solutions in microelectronics.