Photoelectrochemical (PEC) water oxidation using semiconductor oxide films as a working electrode is an essential approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxides-based CuWO₄/BiVO₄/FeCoO_x film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO₄/BiVO₄ photoanode exhibits a significant photocurrent density of 0.82 mA/cm² at 1.23 V vs. RHE under AM 1.5G illumination, corresponding to a record > 380% increase to that of pure CuWO₄ photoanode. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoO_x cocatalyst, resulting in a triple CuWO₄/BiVO₄/FeCoO_x heterojunction with a significant reduction in onset potential and a 500% increase in photocurrent density of pure CuWO₄. Experimental studies and numeric computations were used to provide insights into the photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO₄/BiVO₄ heterojunction, potentially lowering PEC efficiency without external potentials. Conversely, the deposition of the FeCoO_x ultrathin layer over the CuWO₄/BiVO₄ heterojunction induces a - junction on the BiVO₄/FeCoO_x interface, which, when combined with the abundant FeCoO_x oxygen vacancies, results in improved charge separation and transport, as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunctions systems and novel tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.