The goal of this study was to find a way to apply existing constitutive viscoelastic models on ternary polymer blends forming core-shell morphology. An attempt was also made to quantify the interfacial tension through the melt linear viscoelastic properties. High-density polyethylene (HDPE)/polystyrene (PS)/poly(methyl methacrylate) (PMMA) ternary blend system with core-shell morphology was selected as a model. Blends comprising different compositions were prepared via melt mixing. The viscoelastic data required for this research was obtained via small amplitude oscillatory experiments performed on the blend components, binary blends and ternary blend. Several constitutive rheological models including Palierne, Bousmina, Dickie and Jacobs were considered using different approaches for the definition of core-shell as a dispersed phase. The Jacobs model was found to be in good agreement with the experimental results. This could be explained in terms of contribution of additional interfacial tension in the deformation of core-shell droplets, which is more compatible with the assumptions made in the Jacobs model. Nonlinear viscoelastic properties of the samples were also investigated to provide more insights into understanding the role of core-shell (PS/PMMA) droplets on determining the viscoelastic behavior of the ternary blend.