Porous metallic aerogels are a new class of cutting-edge materials useful in catalysis because they combine high conductivity with low density and high surface area. However, the exploration of transition metal-based aerogels with core-shell architectures remains a fundamental challenge. Here, we report a one-step auto-programmed synthesis method to generate a core-shell Cu@Fe@Ni metallic aerogel. Electroactivating (EA) the core-shell Cu@Fe@Ni causes the Fe inner shell to migrate into the Ni outer shell and forms a highly-active catalytic hydroxide on the surface of the aerogel. The resulting EA-Cu@Fe@Ni catalysts exhibited a low OER overpotential of 240 mV at 10 mA cm-2, which is much smaller than bimetallic CuNi (320 mV), CuFe (390 mV), and RuO2 (271 mV). In-situ Raman measurements confirm that the catalyst's outer layer is composed of NiOOH doped with Fe during the electrochemical activation process, resulting in the high OER performance. This work describes the first example of a trimetallic core-shell aerogel synthesized in one step and enables another strategy for designing highly active metals/metal oxide electrocatalysts via surface reconstruction.