This paper extends our previously presented two-dimensional (2-D) Holographic Microwave Imaging Array (HMIA) system for early breast tumour detection to three-dimensional (3-D) imaging, and demonstrates its efficacy using experimental data obtained with a breast phantom. This work describes an experimental setup to collect data to form a 3-D breast image. The obtained experimental result proves that the 3-D HMIA system has potential to become a screening and diagnostic tool that could supplement clinical breast examination through its sensitivity, quantitative record storage, ease-of-use, and inherent low cost.