With the growing demand for high-energy and high-power lithium-ion batteries in electric vehicles and stationary energy storage systems, large-format pouch cells have emerged as a preferred solution due to their high energy density, lightweight construction, and design flexibility. However, their large physical dimensions and the use of localized tab-based current collection introduce significant challenges, particularly in achieving uniform current and potential distribution across the electrodes. These spatial in homogeneities can result in uneven electrode utilization, localized heat accumulation, and accelerated degradation of cell components. This study presents a comprehensive three-dimensional multiphysics simulation of a large-format lithium-ion pouch cell using COMSOL Multiphysics®. The model incorporates realistic geometry, material properties, and electrochemical parameters to investigate the influence of tab placement and collector design under different charging regimes. Fast charging at 4C is shown to induce notable ohmic voltage drops in the current collectors, approximately 5 mV in copper and 9 mV in aluminum, leading to strongly asymmetric lithium intercalation. Current density is initially concentrated near the tabs but progressively shifts toward central regions due to local saturation effects. These findings underline the critical role of cell architecture in dictating performance and reliability. Optimizing tab configuration, increasing collector conductivity, and improving electrode layout are shown to significantly enhance current uniformity, minimize thermal hotspots, and extend battery life. The results demonstrate that 3D multiphysics modeling is a powerful tool for diagnosing internal imbalances and guiding the design of next-generation lithium-ion batteries with improved durability and efficiency.