all four fields into a master photometry list. In the overlapping areas, we selected the duplicates with the smaller total photometric error across the three bands in either field. Finally, we added 2MASS entries to replace failed measurements from saturated sources. Our final near-IR catalog contains a total of 72168 sources.

Mid-infrared Spitzer imaging \[s:observations:ss:mir\]
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We performed a new reduction and photometric extraction of the W3 mosaics from project 1-127 available in the Spitzer Heritage Archive. Mosaics were constructed by processing both the short and long exposure (0.4 and 10.4 sec) Basic Calibrated Sets (BCD) from IRAC (all four bands) and the BCD set from MIPS (24 $\mu$m only) observations using the MOPEX package[^5]. We used the IRAC short exposure datasets to improve extraction of sources down the centers of W3-Main and W3(OH) regions, which present large areas of saturation (increasing with wavelength) due to bright nebulosities in the long exposure sets. However, even at the short exposure images, the bright nebulosity made it very difficult to obtain uniform quality from aperture photometry in the IRAC images, so we performed PSF photometry with `Daophot` on detection lists obtained with `SExtractor`. In this case, we improved the detection efficiency by 