We have derived a mean field Landau-Ginzburg free energy expression for a ternary symmetric homopolymer/diblock copolymer system from a microscopic Edwards Hamiltonian. We then used the time-dependent Landau-Ginzburg equation to model the time evolution of the order parameters after a critical quench into the region of thermodynamic instability in three dimensions. We observe the accumulation of the diblock copolymer at the interface of the domains through expulsion from the interior of the domains. We also see saturation of the diblock copolymers during domain coarsening and consequently the formation of separate, copolymer-rich, domains. We believe these interfacial saturation effects to be a major reason for the slow down of domain growth as the effect is found to be more pronounced at higher diblock copolymer concentrations.