Metal carbido complexes bearing single-carbon-atom ligand such as nitrogenase provide ideal models of adsorbed carbon atoms in heterogeneous catalysis. Trimetallic μ₃-carbido clusterfullerenes found recently represent the simplest metal carbido complexes with the ligands being only carbon atoms, but only few are crystallographically characterized, and its formation prerequisite is unclear. Herein, we synthesize and isolate three vanadium-based μ₃-CCFs featuring V = C double bonds and high valence state of V (+4), including VSc₂C@I_h(7)-C₈₀, VSc₂C@D_5h(6)-C₈₀ and VSc₂C@D_3h(5)-C₇₈. Based on a systematic theoretical study of all reported μ₃-carbido clusterfullerenes, we further propose a supplemental Octet Rule, i.e., an eight-electron configuration of the μ₃-carbido ligand is needed for stabilization of metal carbido clusters within μ₃-carbido clusterfullerenes. Distinct from the classic Effective Atomic Number rule based on valence electron count of metal proposed in the 1920s, this rule counts the valence electrons of the single-carbon-atom ligand, and offers a general rule governing the stabilities of μ₃-carbido clusterfullerenes.