The static and dynamic scattering properties of polystyrene (PS)/poly(dimethylsiloxane) (PDMS)/poly(methyl methacrylate) (PMMA)/toluene solutions are investigated in the conditions of homogeneous single-phase mixtures. The cases of PS/PDMS/toluene and PS/PMMA/toluene were investigated previously in our laboratory, and the purpose of this work is to see the effects of a third polymer in the mixture. The parameters are chosen in such a way that the so-called optical theta condition is fulfilled for the visible components, and the scattered light probes directly the composition fluctuations of the PS/PDMS blend in a matrix of toluene + PMMA. The data show that the presence of PMMA does not change dramatically the qualitative behavior of the scattering properties and affects essentially their values. The results which are obtained independently from static and dynamic measurements are self-consistent. The quasi-elastic data suggest that the hydrodynamic interaction is screened out in the range of concentrations considered in this work. The theoretical framework used to analyze the data is based on the random-phase approximation and is found to be sufficient within the conditions of our experiments as long as the mixtures remain stable in a state of a single homogeneous phase.