Presence of carbon based and/or inorganic fillers in 3D filaments has recently attracted considerable interest from academic and industry to enhance properties of 3D-printed parts. Although 3D-tailored composites have been developed, very little work has been done on the production of advanced 3D filament feedstock for FDM; particularly for highly filled 3D filaments that require (i) high filament strength, (ii) high dimensional accuracy and (iii) superior surface finish. Current FDM filaments rarely exceed filler concentration of 10%, for example, in case of calcium phosphate without sacrificing aforementioned qualities. In this work, a co-axial 3D feedstock filament up to 40% filler concentration has been fabricated via melt spinning; such filament met the desired properties with uniform filler dispersion. We gain further insights on rheological behaviors of such system by performing multiscale flow simulation. On a continuum scale, 2D FEM flow simulations have been conducted to investigate the interfacial instabilities in co-axial flows of dissimilar material. On a mesoscale, we utilize coarse-grained molecular dynamics simulations to predict and investigate the multilayer structure-properties by investigating the effects of i) core/shell layer thickness ratio, ii) filler geometry and concentration, and iii) strain rate.