H − K_s vs. K_s color–magnitude diagram of W3 Main and the chosen subregions in W3 Main. The subregions are circular areas covering the bright Hii regions with a radius of 1′, as shown in Fig 1. The objects marked with a gray circle are identified as K_s-band excess sources using the method described in Sect. 3.4. No field subtraction has been performed in these plots. Note the different plotting range for the IRSN4 panel. Overplotted is the 2 Myr main sequence and PMS isochrone (Brott et al. 2011; Tognelli et al. 2011) for A_Ks= 0 mag (thick dashed line) and A_Ks= 1.5 mag (thin dashed line). The IRSN4 panel shows the masses indicated on the isochrone. The dash-dotted lines are 50% completeness limits for the H- and K_s-bands taken from Table 1. The dotted line represents an estimate of the completeness due to the matching with the J-band. Above this line, the JHK_s matched sample occupies the CMD, below the line, only HK_s matched sources are found.