This study presents the comparison of a coupled electrical-thermal-mechanical finite element model developed to simulate resistance spot welding of S235 steel sheets. The primary objective is to benchmark the numerical simulation against a verified analytical framework for predicting two critical weld features: nugget diameter and heat-affected zone (HAZ) volume. The key advantage of the proposed 3D FEM model over the analytical framework lies in its ability to fully capture three-dimensional asymmetry and complex multi-physics interactions induced by the shunting effect, which cannot be directly predicted by simplified or axisymmetric analytical models. Model predictions were rigorously evaluated across various welding currents and durations, with a specific focus on quantifying the shunting effect (a phenomenon where current diversion through an existing weld nugget compromise subsequent joint quality). The results demonstrate excellent agreement between the numerical and analytical approaches, with predicted nugget diameters and HAZ dimensions deviating by less than ±4% and ±5% from theoretical benchmarks, respectively. The validated model quantifies that shunting can reduce nugget size by over 25% at lower currents (7.5 kA) and, more importantly, reveals a significant lateral HAZ asymmetry of up to 40% under identical conditions, directly attributable to the diverted current path. This strong correlation confirms the FEM model as a robust and reliable predictive tool for optimizing welding parameters and analyzing complex multi-physical interactions in spot welding processes.