Designing highly active, durable, and nonprecious metal-based bifunctional electrocatalysts for overall water electrolysis is of urgent scientific importance to realize the sustainable hydrogen production, which remains a grand challenge. Herein, an innovative approach is demonstrated to synthesize flower-like 3D homogenous trimetallic Mn, Ni, Co phosphide catalysts directly on nickel foam via electrodeposition followed by plasma phosphidation. The electrochemical activity of the catalysts with varying Mn:Ni:Co ratios is assessed to identify the optimal composition, demonstrating that the equimolar trimetallic phosphide yields an outstanding HER catalytic performance with a current density of 10?mA?cm?2 at an ultra-low overpotential of ~14?mV, outperforming the best reported electrocatalysts. This is asserted by the DFT calculations, revealing strong interaction of the metals and the P atom, resulting in enhanced water activation and optimized GH* values for the HER process. Moreover, this optimal composition appreciably catalyzes the OER by exposing more intrinsic active species in-situ formed on the catalyst surface during the OER. Therefore, the Mn1-Ni1-Co1-P-(O)/NF catalyst exhibits a decreased overpotential of ~289?mV at 10?mA?cm?2. More importantly, the electrocatalyst sustains perfect durability up to 48?h at a current density of 10?mA?cm?2 and continued 5000 cycling stability for both HER and OER. Meanwhile, the assembled MNC-P/NF||MNC-P/NF full water electrolyzer system attains an extremely low cell voltage of 1.48?V at 10?mA?cm?2. Significantly, the robust stability of the overall system results in a remarkable current retention of ~96% after a continuous 50-h run. Therefore, this study provides a facile design and a scalable construction of superb bifunctional ternary MNC-phosphide electrocatalysts for efficient electrochemical energy production systems.