The phase behavior and dimensional scaling of symmetric ternary blends composed of poly(styrene-block-methyl methacrylate) block copolymers and the corresponding polystyrene and poly(methyl methacrylate) homopolymers in thin films were investigated as a function of chi N (the product of the Flory-Huggins interaction parameter and the overall degree of polymerization of the block copolymer), alpha (the ratio of the degree of polymerization of the homopolymers to that of the block copolymer), and phi(H) (the volume fraction of homopolymers in the blends). The phase transitions were characterized by three methods: disappearance of high-order peaks in the fast Fourier transform (FFT) spectra from scanning electron microscopy (SEM) images, application of the amphiphilicity factor determined from the FFTs, and, in the case of thicker films, the appearance of either microphase- or macrophase-separated domains in SEM images. Above the order-disorder transition (ODT) in the chi N-phi(H) phase diagram, the symmetric ternary blends transitioned from lamellae to a microemulsion and then to macrophase-separated domains as phi(H) increased. The phase transitions depended weakly on chi N and alpha in the range of 12.7 <= chi N <= 36.7 and 0.20 <= alpha <= 0.99. The periods of swollen lamellae and microemulsions (L-B) were determined as a function of phi(H) and a and were well described empirically with L-B = L-0/(l-phi(H))(beta), where L-0 is the natural period of pure block copolymer and beta is a parameter that depends linearly on alpha with values ranging from similar to 0.5 to 1.5.