We propose a three-dimensional (3D) microwave-induced thermo-acoustic imaging technique for early-stage breast cancer detection, which offers a high imaging contrast and high spatial resolution. The robust Capon beam-forming (RCB) algorithm, which is a data-adaptive approach, is considered for signal processing and image formation. The finite-difference time-domain (FDTD) method is applied to electromagnetic simulation as well as acoustic simulation. The feasibility of this technique is demonstrated via a numerical example based on a 3D breast model.