Abstract: A full three-dimensional numerical study on the electro-convection of dielectric liquids contained in a cubical cavity is reported. All boundaries are solid walls. The four lateral sides are electrically insulating and the top and bottom walls are planar electrodes. The flow motion is driven by the volumetric Coulomb force exerting on the free charge carriers introduced by a strong unipolar injection from the bottom electrode. The charge injection takes place due to the electro-chemical reaction at the interface between liquid and electrode. The unsteady Navier-Stokes equations and a reduced set of Maxwell's equations in the limit of electroquasistatics are solved using an efficient finite volume method with 2^\rm nd order accuracy in space and time. Considering the strong convection-dominating nature of the charge conservation equation, a total variation diminishing scheme is specially used to solve this equation in order to obtain physically-bounded and accurate solution. It is found that the flow is characterized by a subcritical bifurcation in the finite amplitude regime. A linear stability criterion and a nonlinear one, which correspond respectively to the onset and stop of the flow motion, are numerically determined. Since the nonlinear criterion is smaller than the linear one, there exists a hysteresis loop. Compared to the free convection in the infinitely large domain case, the constraint imposed by the lateral walls dramatically changes the flow structure and increases the two criteria. In addition, the spatial features of charge density distribution and velocity field are discussed in details. A central region free of charges is observed. This void region is formed due to the competition between the fluid velocity and the drift velocity, and it is closely related to the subcritical bifurcation feature of the flow. In addition, computations are also performed with a case with smaller domain sizes, and the results show that the linear bifurcation of the flow is supercritical. Once the system losses its linear stability, a steady convection state without charge void region is reached. The present results extend previous research on the two-dimensional electro-convection in confined cavities, and they provide reference for the three-dimensional theoretical analysis of the linear and weakly nonlinear stability.