In order to simulate three-dimensional dendritic evolution of binary alloys under melt convection, a modified cellular automaton (MCA) model was developed. With considering the influence of surface energy anisotropy and solute diffusion on the evolution of solid/liquid interface, and solving the mass transport equation coupled with Navier-Stokes equations on the same grid, the MCA model could simulate the interaction between solute diffusion and melt convection. In this study, 3-D dendritic growth of Al-7wt%Si alloy was simulated with forced convection at constant undercooling by the MCA model. It is found that the upstream tip of the dendrite grows faster than that of the downstream tip under forced convection, due to lower solute concentration and larger solute gradient in the front of the upstream interface caused by forced convection. Meanwhile, the simulated dimensionless solute supersaturation agrees well with Oseen-Ivantsov solution at higher forced convection. The effect of forced convection on dendrite morphology was also studied under 3-D and 2-D simulation. The solute enriched at the upstream frontier of solid/liquid interface is washed away bypassing the primary dendrite arms from the upstream to downstream direction by 3-D forced convection. However, in 2-D convection, the solute could only go over the tips of the perpendicular dendrite arms from the upstream to downstream direction. Therefore, the 3-D MCA model is more accurate in simulating the influence of forced convection on dendritic growth than a 2-D model.