Wave breaking has large impact on stresses and loading on marine structures, but it is not yet accounted for in the design process. A numerical investigation is here presented to fully assess the three-dimensional velocity field underneath a breaking wave. The breaking onset is achieved by modulating an initial monochromatic wave with infinitesimal sideband perturbations. The latter triggers a nonlinear energy transfer, which allows one individual waves to grow and break once the steepness has overcome a specific threshold. Numerical simulations of the Navier-Stokes equations are carried out by means of the open source CFD code OpenFOAM. To speed up the simulation process, the nonlinear evolution of the perturbed Stokes wave is first obtained with a High-Order Spectral Method (HOSM) until the onset of breaking; surface elevation and velocity field are then transferred to the CFD for the final stage of the breaking process. The fully three-dimensional turbulent kinematic field is presented and discussed with reference to the velocity field predicted by the theory.