During high-dose-rate (HDR) interstitial brachytherapy of gynecologic malignancies, precise placement of multiple needles is necessary to provide optimal dose to the tumor while avoiding overexposing nearby healthy organs, such as the bladder and rectum. Needles are currently placed based on preoperative imaging and clinical examination but there is currently no standard for intraoperative image guidance. We propose the use of a three-dimensional (3D) ultrasound (US) system incorporating three scanning geometries: 3D transrectal US (TRUS), 360° 3D sidefire transvaginal US (TVUS), and 3D endfire TVUS, to provide an accessible and versatile tool for intraoperative image guidance during interstitial gynecologic brachytherapy. Images are generated in 12 - 20 s by rotating a conventional two-dimensional US probe, providing a reconstructed 3D image immediately following acquisition. Studies of needles in patient images show mean differences in needle positions of 3.82 ± 1.86 mm and 2.36 ± 0.97 mm in TRUS and sidefire TVUS, respectively, when compared to the clinical x-ray computed tomography (CT) images. A proof-of-concept phantom study of the endfire TVUS mode demonstrated a mean positional difference of 1.91 ± 0.24 mm. Additionally, an automatic needle segmentation tool was tested on a 360° 3D TVUS patient image resulting in a mean angular difference of 0.44 ± 0.19 ° and mean positional difference of 0.78 ± 0.17 mm when compared to manually segmented needles. The implementation of 3D US image guidance during HDR interstitial gynecologic brachytherapy provides a versatile intraoperative system with the potential for improved implant quality and reduced risk to nearby organs.