Three-dimensional printed (3DP) Polyethylene Terephthalate Glycol (PETG) ternary composites reinforced with short carbon fibers (sCF) and multi-walled carbon nanotubes (CNT) are reported for the first time. A single-screw extrusion process is employed to manufacture micro-/nano- ternary PETG-based filaments for Fused Filament Fabrication (FFF)-3DP. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Differential Scanning Calorimetry (DSC) and Raman spectroscopy are conducted to characterize the material’s physico-chemical properties at filament level. Mechanical and electrical characterizations of 3D-printed PETG composites with hierarchical micro-/nano-filler networks revealed substantial improvements over the neat polymer. Rheological characterization of the composites was conducted in the melt state. The incorporation of CNT significantly altered the melt polymer chain entanglement and the composite spanning network, resulting in changes in molecular entanglement, plateau modulus and yielding behavior, which ultimately affected the 3D printing process and the final composite’s performance. Tensile and flexural tests were performed for unidirectional: (UD[0]16) and cross-ply: (CP[0/90]16s) “16-ply laminate” 3DP specimens, accompanied with fractographic analyses. The highest values in tensile modulus (7.93±0.38 GPa) and strength (53.3 ± 3.49 MPa) are found for PETG loaded with 15.0 wt.% sCF and 2.0 wt.% CNT, hereafter denoted as PETG/sCF(15)/CNT(2), with 285% and 40.5% increase, respectively, compared to neat PETG. The multi-scale PETG/sCF(15)/CNT(2) composite exhibits a 936% improvement in electrical conductivity (σ) compared to PETG/CNT(5) nanocomposite demonstrating the synergy of the co-existing nano-/ micro-fillers’ network within the polymer matrix. The overall results indicate a promising multi-scale reinforced feedstock material with superior mechanical performance and inter-/intralaminar bead adhesion in the 3DP manufactured specimens. The optimized formulations obtained herein could be applied in other polymer matrices towards 3DP multi-functional structural composites with enhanced mechanical and electrical properties.