The dynamic mean-field density functional method is used to describe phase separation including hydrodynamic effects in specific three-dimensional (3D) compressible copolymer liquids. We show that it is justified to use Darcy's approximation for the velocity field. This simple model enables us to reproduce both the increased domain growth and the faster removal of defects that are a result of hydrodynamics. We perform a simulation of a diblock copolymer melt to study the viscous effects in 3D. The free energy is shown to decrease twice as fast as in a simulation which only includes diffusion. The hydrodynamics are shown to become important only in the later stages of phase separation as is also predicted from theoretical analysis. The separation process proceeds faster because the growth of bulk domains is accelerated compared to the purely diffusive case.