  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/ACIS observations of 14
Galactic MSFRs and surrounding fields. The MOXC3 MSFRs are NGC 2264, NGC 6193,
RCW 108-IR, Aur OB1, DR15, NGC 6231, Berkeley 87, NGC 6357, AFGL 4029, h Per
(NGC 869), NGC 281, Onsala 2S, G305, and RCW 49 (Wd 2); they have distances of
0.7 kpc to 4.2 kpc. Most exhibit clumped or clustered young stellar
populations; several contain at least two distinct massive young stellar
clusters. The total MOXC3 catalog includes 27,923 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.
