Tailoring trimodal polyethylene (PE) molar mass distributions by means of ethylene polymerization on three-site catalysts, supported on functionalized graphene (FG), enables nanophase separation during polymerization and melt processing, paralleled by PE self-reinforcement. Typically, FG/MAO-supported three-site catalysts combine bis(iminopyridyl)chromium tricbloride (CrBIP), producing PE wax having high crystallization rate, and quinolylcyclo-pentadienylchromium dichloride (CrQCp), forming in situ ultrahigh molecular weight PE (UHMWPE) nanostructures, with bis(iminopyridyl)iron dichloride (FeBIP) or bis(tert-butyl cyclopentadienyl)zirconium (ZrCp), respectively, producing HDPE with variable intermediate molar mass. During injection molding, the formation of shish-kebab fiber-like extended-chain UHMWPE structures, as verified by SEM, AFM, and DSC, account for effective self-reinforcement. Only in the presence of high UHMWPE content, PE wax, usually an unwanted byproduct in HDPE synthesis, functions as a built-in processing aid and enables the incorporation of much higher UHMWPE contents (30 wt %) than previously thought to be tolerable in injection molding. Whereas the incorporation of UHMWPE/PE wax blends improves stiffness and strength, the simultaneous FG dispersion accounts for substantially higher impact strength.