We theoretically predict phase behavior of ternary blends of two different homopolymers (denoted by A and B) and a diblock copolymer consisting of A- and B monomers. Using a mean-field approach of Landau formalism, the phase diagrams are constructed, and the stability of various microstructures, the biphasic coexistence and the triphasic coexistence comprising ordered phases, are discussed. Our theoretical predictions show that swelling of microphase can be observed only when the molecular weight of added homopolymers is comparable to that of the corresponding block of the copolymer, whereas the addition of sufficiently smaller or larger homopolymers leads to a "dilution" of a ordered phase or various phase coexistence between ordered and disordered phase, respectively.