In this work, we developed tri-metallic 3D NiFeM nanofoam with unique metallic glass shell and crystalline mix-phase core structures for application in water electrolyzer. The NiFeMn delivered a remarkably enhanced current density of 2.0 A cm^-2 at 1.7 V by flowing with 0.1 M KOH and NiFeCu catalysts afforded a very promising current density of 1.2 A cm^-1 at voltage of ~2.3 V in full cell water electrolyzer tests by flowing with pure water. Introducing a third transition metal enhances the intermediate adsorption energy of the NiFe hydroxide active sites by modification of their local electron structure, thus effectively improving their catalytic activity exhibited by increased current density at higher potential. Furthermore, the inner crystalline core made a great contribution in promoting the electron donating process, which is beneficial in accelerating the reaction process especially in water electrolyte. Besides, benefiting from the self-supported as well as meso- and macro-porous structure, these 3D catalysts have high corrosion resistance at higher potential which significantly alleviates the carbon corrosion issues using carbon-based OER catalysts. Therefore, the NiFeM catalysts display great potential in practical applications beyond alkaline water electrolyzers.