Copper foam (CF)‐supported 3D nanosheets array composed of ternary Ni−Co−Cu chalcogenides are prepared by a simple in situ formation process. Specifically, a highly electroactive Ni−Co binary sulfide in nanosheets is synthesized against the CF backbone, whereas the copper species migrate from the CF to the Ni−Co nanosheets, leading to the in situ formation of the ternary metallic sulfide (Ni−Co−Cu−S, NCCS). Due to the synergistic interaction of Ni, Co, and Cu sulfides confined in nanosheets, this NCCS material demonstrates good mechanical robustness, a large surface area, and enhanced electric conductivity. As a result, the NCCS exhibits a high specific capacitance (750 mF cm−2 at 100 mA cm−2) with good cycling performance (97.14% after 10 000 cycles) when used as supercapacitor electrodes. In addition, the 3D porous hierarchical nanostructure of NCCS provides nanoconfined water molecule channels to achieve high‐yield solar steam generation, delivering an enhanced evaporation rate of 2.48 kg m−2 h−1 under 1 sun irradiation.