Phase separated structure of ternary blends of A and B homopolymers and symmetric AB diblock copolymer is investigated using a lattice (real-space) self-consistent field theory. This paper includes the detailed description of our published results [Kodama, Komura, and Tamura, Europhys. Lett. 53, 46 (2001)] as well as more extended calculations. We consider the symmetric case, namely, (i) both A and B homopolymers have the same degree of polymerization N-A=N-B; (ii) AB diblock copolymer of length N-AB is symmetric; (iii) average volume fractions of A and B homopolymers are equal. We looked into the influence of relative chain lengths alpha=N-A/N-AB on the phase separated structure. Our numerical simulations are performed in the real space without assuming the symmetry of the structure a priori. For the fixed copolymer length and alpha<1, the typical length scale of the microphase separated structure become smaller for relatively shorter homopolymer chains (small alpha). In other words, the homopolymers becomes more efficient to swell the microphase separated structure for longer homopolymer chains (large alpha). Detailed free-energy analysis revealed that the stability of the lamellar phase is marginal for small block copolymer volume fraction. For alpha>1, on the other hand, three-phase coexistence either between the disorder, A-rich and B-rich phases or between the lamellar, A-rich and B-rich phases is observed.