Nickel-, cobalt-, and iron-based (oxy)hydroxides comprise the most-commonly studied electrocatalysts for the oxygen-evolution reaction (OER) in alkaline solution. A fundamental understanding of composition-structure-activity relationships for mixed-metal Ni-Co and Ni-Co-Fe (oxy)hydroxides is important to guide the design of advanced OER catalysts. Here we use cyclic voltammetry, chronopotentiometry, inductively-coupled plasma-optical emission spectroscopy, and in situ electrical conductivity measurements to characterize the properties and activity of various compositions of Ni-Co-Fe (oxy)hydroxides prepared by cathodic co-electrodeposition. Consistent with previous studies, we find Fe is essential for the mixed-metal (oxy)hydroxides to achieve high OER activity. In the rigorous absence of Fe (achieved by using specially cleaned electrolytes), the most-active Ni-Co (oxy)hydroxide composition has an OER turn-over frequency only twice that of pure Co (oxy)hydroxide, suggesting minimal synergism between the two metals. The addition of Co to Ni-Fe (oxy)hydroxides shifts the onset of electrical conductivity to lower potentials, but has little effect on the intrinsic OER activity, with the most-active Ni-Co-Fe (oxy)hydroxide having an OER turn-over frequency only ~ 1.5 times that of the Ni-Fe (oxy)hydroxides. The magnitudes of the electrical conductivities are similar for all the compositions measured. Density-functional-theory-calculated projected density of states show a significant contribution of all chemical elements at the valence band edge of the mixed-metal oxyhydroxide electronic structure, demonstrating significant electronic hybridization between the elements. The calculations suggest the involvement of all the elements in modulating the electronic structure at putative Fe-based active sites that are probably located at edges or defects in the two-dimensional oxyhydroxide sheets.