Electrochemical nitrate reduction offers a sustainable route to produce ammonia while simultaneously remediating nitrate pollution. Here, we report a series of trimetallic catalysts derived from carbonized zeolitic imidazolate frameworks (Czif), incorporating Zn and Cu into a Co-based metal-organic framework (MOF) scaffold. Among them, Czif-Zn₃Cu₁ (zinc and copper precursors at a molar ratio of Zn:Cu = 3:1) exhibits the highest Faradaic efficiency (> 90%) and NH₃ yield rate across a broad current density range (100–500 mA/cm²), outperforming both undoped and bimetallic counterparts. Structural characterization reveals the preservation of MOF morphology, with uniformly dispersed Co, Zn, and Cu sites embedded in a porous N-doped carbon matrix. The optimized Zn:Cu ratio enhances intermediate stabilization and suppresses competing hydrogen evolution, supported by a comprehensive set of analyses. Operando flow-cell tests confirm the catalyst’s energy efficiency, nitrate tolerance (10–1000 mM), and long-term durability over 100 h. Density functional theory (DFT) calculations confirm that the trimetallic synergy of Czif-Zn₃Cu₁ lowers the overall energy barrier and underpins its enhanced activity. This work highlights the importance of rational trimetallic design and MOF-derived architectures in achieving high-performance electrocatalysts for selective and scalable nitrate-to-ammonia conversion.