BACKGROUND: Inter-fractional and intra-fractional positioning and motion monitoring based on implanted fiducials through kilovoltage (kV) imaging is a cost-effective approach to enhance intact prostate radiotherapy treatment accuracy. However, comprehensive studies for three-dimensional (3D) position extraction and the corrective action threshold are lacking. PURPOSE: To develop and verify a fiducial motion monitoring system with 3D position information based on triggered kV images taken during treatment delivery, and to comprehensively evaluate the recommended thresholds for corrective action during treatment. METHODS: An in-house fiducial triangulation algorithm was developed to monitor fiducial positions using subsequent kV triggered images. The precision of fiducial triangulation and motion detection was validated on a pelvis phantom with four fiducial inserts. A retrospective study was conducted on prostate cancer patients who received either moderately hypofractionated radiotherapy (MHRT, n = 16) or stereotactic body radiotherapy (SBRT, n = 12), categorized by endo-rectal balloon (ERB) use. Intra-fractional positions relative to isocenter were computed, and these positions were analyzed in comparison to the original marker locations of the patient's treatment plan. A linear regression fit was used per fraction to determine correlation coefficients between motions in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions. Average fraction time (AFT) of treatment was reported based on the observed average time spent on steps in the workflow and the percentage of fiducials caught outside of the current two-dimensional (2D) tolerance threshold of 5 mm. The observed 3D offsets were used to estimate AFT for various 3D tolerance thresholds. RESULTS: Overall, our triangulation method proved to be very precise for static cases, where phantom measurements revealed a fiducial position triangulation precision of CONCLUSIONS: Our study showcases a promising subsequent kV-based triangulation method for intra-fractional prostate motion monitoring. Acquiring 3D motion information results in higher out-of-tolerance catch-rates particularly in the depth dimension of the kV images, which is perpendicular to the treatment beam. Failure to properly observe and catch these offsets would result in sub-optimal conformality and accuracy of the dose delivery.