Simple three-layer broadband optical absorbers have attracted considerable attention because of their potential applications in solar-energy harvesting, thermal emitters, and high-performance optical sensing systems. In particular, broadband absorbers can significantly enhance light–matter interaction, making them promising candidates for improving the performance of optical sensors and ultraviolet photodetectors. In this study, a simple three-layer optical absorber composed of a W ground plane, an Al2O3 dielectric spacer, and a top W square resonator containing a centrally hollow cylindrical cavity was designed and analyzed using the finite element method implemented in COMSOL Multiphysics. The effects of structural parameters, including the thicknesses of the W and Al2O3 layers, the dimensions of the top resonator, and the radius of the hollow cylindrical cavity, were systematically investigated to optimize the broadband absorption performance. In addition, optical impedance, electric field distribution, magnetic field distribution, and angular absorption characteristics under both transverse electric and transverse magnetic polarizations were analyzed to clarify the underlying absorption mechanisms. The optimized absorber exhibits absorptivity exceeding 90% over the wavelength range of 200–1760 nm, together with excellent angular stability under oblique incidence. Owing to its simple structure, broadband absorption capability, and robust optical performance, the proposed design is a promising candidate not only for solar-energy harvesting but also for broadband optical sensors, ultraviolet photodetectors, and other integrated optoelectronic sensing devices.