This study presents a detailed three-dimensional (3D) numerical analysis of Vertical Axis Wind Turbines (VAWTs) to investigate aerodynamic performance and 3D flow effects. Building upon a validated two-dimensional (2D) model, the 3D computational domain leverages symmetry boundary conditions to reduce computational costs, modelling only half of the rotor blades. The 3D domain's dimensions and grid resolutions are in alignment with previous 2D studies, with modifications for span-wise and blade tip regions. A hybrid turbulence model, SBES with Transition SST, is employed, yielding moment coefficient predictions within an acceptable discrepancy range compared to experimental data. Grid independence is established via systematic grid refinement in span-wise and far-field sub-domains, ensuring accurate representation of 3D flow characteristics. Results highlight significant deviations in aerodynamic performance along span-wise positions, particularly at azimuthal ranges of 45o-150o and 210o-270o respectively, attributed to dynamic stall, vortex interactions, and blade tip effects. While the 3D CFD model underpredicts power coefficients compared to experimental data, it captures three-dimensional flow phenomena absent in 2D simulations. Contour plots of span-wise vorticity reveal consistent blade tip vortex formation and dissipation across azimuthal positions, underscoring the reduced aerodynamic impact beyond the tip. The study confirms the need for refined turbulence models and additional experimental validation to enhance 3D VAWT performance prediction.