Atomically precise heterogeneous catalysts offer unprecedented opportunities for establishing clear structure-activity relationships in catalysis. However, the precise control over the number of metal atoms in such systems remains a significant challenge, primarily due to difficulties in maintaining atomic-level dispersion, preventing metal migration and aggregation. This study develops a tandem synthesis strategy combining ligand protection and precursor engineering. First, atomic-level precision precursors Pt₁(C₄₄H₃₂P₂)₂(CO)₄, Pt₂(C₁₈H₁₂Cl₃P)₃Br₂(CO)₁ and Pt₃(C₁₈H₂₁P)₅(CO)₂ are synthesized using mixed ligands. Subsequently, by integrating Joule induction rapid thermal annealing technology, structurally well-defined Pt₁/OKB, Pt₂/OKB, and Pt₃/OKB catalysts are successfully constructed on oxidized Ketjenblack (OKB) support. In mild conditions (60 °C, 0.5 MPa H₂) for nitrobenzene selective hydrogenation, the Pt₃/OKB catalyst achieves>99% conversion and>99% aniline selectivity, significantly outperforming Pt single-atom, Pt dual-atom, and Pt nanoparticle catalysts. Density functional theory (DFT) calculations demonstrate that the unique coordination environment in the triatomic structure optimizes adsorption strength, suppressing excessive benzene ring hydrogenation while facilitating H₂ activation and intermediate stabilization, thereby achieving synergistic regulation of activity and selectivity at the atomic scale. This study provides an atomically precise synthetic method for heterogeneous catalysts and elucidates the dependence of catalytic hydrogenation performance on the number of metal atoms.