In this work, the catalytic mechanisms of the nitrogen reduction reaction (NRR) over unsupported and sumanene(C₂₁H₁₂) supported trimetallic late transition metal clusters M₃(M = Fe, Co, Ni) and the noble metal cluster Rh₃ were systematically investigated via density functional theory (DFT) calculations, with comparative analysis against early transition metal cluster X₃(X = Ti, Zr, V, Nb) systems. The results show that the C₂₁H₁₂ support can modulate the electronic distribution of the clusters and enhance the activation ability of the N≡N bond, rendering the NRR activity of C₂₁H₁₂-M₃ superior to that of C₂₁H₁₂-X₃; notably, the non-noble metal system C₂₁H₁₂-Co₃ surpasses the noble metal system C₂₁H₁₂-Rh₃ in NRR activity after being loaded on C₂₁H₁₂. For the unsupported late transition metal clusters, the rate-determining step (RDS) of NRR is the first hydrogenation step (^*N₂→^*NNH). C₂₁H₁₂ optimizes the local charge environment, reducing the Gibbs free energy barrier of RDS (ΔG_RDS) of C₂₁H₁₂-M₃ by 0.16 - 0.92 eV, among which C₂₁H₁₂-Co₃ exhibits a mere △G_RDS of 0.64 eV via the enzymatic mechanism (EM) pathway. C₂₁H₁₂ shows better compatibility with late transition metal clusters, where the performance enhancement is dominated by the preferential reduction of △G_RDS. This study also reveals an approximately linear positive correlation between the HOMO energy level of the catalysts and the ΔG of the final hydrogenation step, and verifies that the electronic smearing effect of C₂₁H₁₂ enables the efficient replacement of noble metals with non-noble metals. Innovatively, we elucidate the differential modulation mechanism of the sumanene support on different metal clusters and establish a correlation between the intrinsic properties of catalytic materials and reaction energy barriers, providing new theoretical insights for the design of high-efficiency NRR electrocatalysts. Future research can expand to heteronuclear cluster systems, optimize catalytic performance by introducing external electric fields, and carry out experimental synthesis and verification to promote the transformation of theoretical results into practical applications.