The design of hybrid nanostructures composed of plasmonic metals and semiconductor oxides plays a major role in determining their efficiency for the conversion of solar energy. In this work, the light-harvesting properties of spherical core–shell hybrid nanostructures composed of a plasmonic metal core (Au, Ag, and Al) coated by a magnetite (Fe3O4) shell have been investigated through systematic discrete dipole approximation simulations. The diameter of the plasmonic core D was varied in the range of 5–90 nm, while the thickness of the Fe3O4 shell S was varied in the range of 2–40 nm. It was found that for a given set of D and S values, the absorbed photon flux within the Fe3O4 shell, ϕ, increases in the order Al, Au, and Ag. Furthermore, for a given size, which is D + 2S = constant, the largest ϕ value is approximately achieved when D/S = 3, 4, and 5 for Al, Au, and Ag as the core material, respectively. In addition, it was empirically found that ϕ correlates directly with the predictor K, a quantity that depends on D, S, and the resonance energy of the plasmon. The results presented contribute to expanding the tool kit that allows optimizing the design of hybrid nanostructures in order to improve their photoactive properties.