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[Display omitted] • The graphitic carbon nitride (g-C 3 N 4) shows unique geometrical characterizes to anchoring metal trimer clusters. • They have excellent catalytic performances for yielding NH 3 products with limiting potential of 0.02–0.50 V involving kinetic barrier of 0.03–0.46 eV. • The establish the relationship between surficial activities and electronic properties. As a cleaner and more energy-efficient alternative, the electrochemical reduction of N 2 to NH 3 under mild conditions has emerged as a promising avenue for environmental protection. However, developing catalysts with both high activity and selectivity has been a challenging task. The advent of atomically dispersed catalysts is revolutionizing the field of catalysis. Drawing inspiration from the successful fabrication of Ru trimer on graphitic carbon nitride (g-C 3 N 4) nanosheets, herein, we adopted a series of trimeric metal clusters anchored on g-C 3 N 4 monolayer (TM 3 @g-C 3 N 4 , TM = 3 d , 4 d and 5 d transition metal atom), as well as utilized the criteria range to describe the stability, activity, energy cost and selectivity of supported trimers for N 2 reduction reaction based on ab initio calculations combined with multi-level screening strategies. Finally, 5 candidate TM 3 @g-C 3 N 4 (TM = Rh, Ru, V, Mo, and Re) exhibits decent catalytic activity with limiting potential ranging of –0.50 – –0.02 V. These spatially confined triatomic metal centers demonstrated proficiency in activating N 2 molecules while exhibiting tunable surficial reactivity through modulation of the metal elements and interfacial electron coupling between trimers and underlying substrates. More importantly, a structure–activity relationship was established, which provides crucial insights for precise design of highly efficient atomically-precise catalysts at an atomic level for hydrogen fuel storage. [ABSTRACT FROM AUTHOR]