A design for a highly reconfigurable intelligent surface (RIS) that can provide three-dimensional passive reflective beamforming suitable for sub-6 GHz frequency bands is proposed. The RIS is based on a novel double-layer structure consisting of independently controllable and highly reconfigurable elements on top of a ground plane. The novelty of the structure is the use of reconfigurable RIS elements, whose size is of the order of a wavelength, that are optimized to function over wide incident and reflected angles. In addition, the individual elements each have four RF switches, in a 5x5 sub-element structure, providing 16 different reflective phases, from 0 to 360 degrees relative to the incident waves. To quantify the performance of the RIS, the performance metric of phase entropy is defined and it is also utilized as an objective function for optimizing the design. An efficient analytical method for predicting the reflected waves from the RIS elements, as well as phase entropy, is also provided. For verification, a prototype of a 4x4-element RIS, with a total size of 0.32x0.34 m², is fabricated and an experimental setup for measuring the scattered pattern is described. It is shown that waves incident from various angles can be passively beam steered to any angle within 0-360 degrees azimuth and 70 degrees elevation angles. The high reconfigurability to manipulate incident waves makes the proposed RIS a promising surface for use in future high-capacity communication systems.