  In this work we have carried out an in-depth analysis of the young stellar
content in the W3 GMC. The YSO population was identified and classified in the
IRAC/MIPS color-magnitude space according to the `Class' scheme and compared to
other classifications based on intrinsic properties. Class 0/I and II
candidates were also compared to low/intermediate-mass pre-main-sequence stars
selected through their colors and magnitudes in 2MASS. We find that a reliable
color/magnitude selection of low-mass PMS stars in the infrared requires prior
knowledge of the protostar population, while intermediate mass objects can be
more reliably identified. By means of the MST algorithm and our YSO spatial
distribution and age maps we investigated the YSO groups and the star formation
history in W3. We find signatures of clustered and distributed star formation
in both triggered and quiescent environments. The central/western parts of the
GMC are dominated by large scale turbulence likely powered by isolated bursts
of star formation that triggered secondary star formation events. Star
formation in the eastern high density layer also shows signs of extended
periods of star formation. While our findings support triggering as a key
factor for inducing and enhancing some of the major star forming activity in
the HDL (e.g., W3 Main/W3(OH)), we argue that some degree of quiescent or
spontaneous star formation is required to explain the observed YSO population.
Our results also support previous studies claiming an spontaneous origin for
the isolated massive star(s) powering KR 140.
