Cooperative photocatalysis for simultaneous hydrogen peroxide (H₂O₂) production and organic oxidation presents a sustainable approach to solar-to-chemical energy conversion. While Zn_xIn₂S_x+3 materials are promising candidates, the study of high stoichiometry (x > 1) variants is still limited. Moreover, the synergy of mixed-phase C₃N₅ and Zn_xIn₂S_x+3 remains to be explored. Herein, a series of Zn_xIn₂S_x+3 (x = 1–4) photocatalysts (RZISx) were synthesized via reflux, with Zn₃In₂S₆ (x = 3) identified as the optimal composition, achieving H₂O₂ and benzaldehyde production rates of 1430.9±65.6 μM h⁻¹ and 2473.2±264.2 μM h⁻¹, respectively. Crystalline C₃N₅ (CCN550) was synthesized by the molten-salt method and a ternary heterostructure (5CCN/RZIS3) was further developed by compositing Zn₃In₂S₆ with CCN550 during refluxing, leading to enhanced yields of H₂O₂ (2655.2±167.1 μM h⁻¹; apparent quantum efficiency (AQE): 1.1% at 420 nm) and benzaldehyde (2685.9±126.5 μM h⁻¹). Improved performance stems from the intimate interface between Zn₃In₂S₆ and heptazine/triazine domains in C₃N₅, which facilitates efficient charge separation and boosts O₂ adsorption. Rotating disk electrode measurements confirmed high selectivity toward the two-electron oxygen reduction pathway. This study introduces a ternary heterojunction strategy and provides insights into the role of crystalline structure and heterointerface engineering in advancing dual-functional photocatalysis.