We offer to the star formation community the third installment of the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog (MOXC3), a compilation of X-ray point sources detected in 50 archival Chandra Advanced CCD Imaging Spectrometer observations of 14 Galactic MSFRs and surrounding fields. The MOXC3 MSFRs are NGC2264, NGC6193, RCW108-IR, AurOB1, DR15, NGC6231, Berkeley87, NGC6357, AFGL4029, hPer (NGC869), NGC281, Onsala2S, G305, and RCW49 (Wd2); they have distances of 0.7-4.2kpc. Most exhibit clumped or clustered young stellar populations; several contain at least two distinct massive young stellar clusters. The total MOXC3 catalog includes 27923 X-ray point sources. We take great care to identify even the faintest X-ray point sources across these fields. This allows us to remove this point source light, revealing diffuse X-ray structures that pervade and surround MSFRs, often generated by hot plasmas from massive star feedback. As we found in MOXC1 and MOXC2, diffuse X-ray emission is traceable in all MOXC3 MSFRs; here we perform spectral fitting to investigate the origins of selected diffuse regions. Once again, MOXC3 shows the value of high spatial resolution X-ray studies of MSFRs enabled by Chandra.