Ultrasound computed tomography (USCT) is a promising imaging technique for breast cancer diagnosis. Full-waveform inversion (FWI)-based image reconstruction methods based on the acoustic wave equation can produce quantitatively accurate images of acoustic properties with higher spatial resolution compared to ray-based methods. One common USCT design employs a circular ring-array of elevation-focused ultrasonic transducers that can be translated orthogonally to the imaging plane to achieve volumetric scanning. Currently, slice-by-slice (SBS) reconstruction methods are often used in this setting. Such methods use a 2D wave physic model to reconstruct a cross-sectional image for each position of the ring-array and vertically stack them together to render the 3D volume. However, this approach neglects the 3D wave propagation physics and transducer’s focusing properties, leading to out-of-plane scattering artifacts and degraded spatial resolution. To address this, a 3D time-domain FWI method that utilizes measurement data acquired at multiple adjacent ring-array locations is proposed. The reconstruction method was investigated using virtual imaging studies of ring-array-based USCT that employed realistic 3D numerical breast phantoms. The results demonstrated improved spatial resolution (both in-plane and off-plane) and reduced image artifacts compared to the SBS approach. Additionally, the impact of the number of ring-array locations on image quality is assessed.