  We present a multi-wavelength analysis of the history of star formation in
the W3 complex. Using deep, near-infrared ground-based images, combined with
images obtained with Spitzer and Chandra observatories, we identified and
classified young embedded sources. We identified the principal clusters in the
complex, and determined their structure and extension. We constructed
extinction-limited samples for five principal clusters, and constructed K-band
luminosity functions (KLF) that we compare with those of artificial clusters
with varying ages. This analysis provided mean ages and possible age spreads
for the clusters. We found that IC 1795, the centermost cluster of the complex,
still hosts a large fraction of young sources with circumstellar disks. This
indicates that star formation was active in IC 1795 as recently as 2 Myr ago,
simultaneous to the star forming activity in the flanking embedded clusters,
W3-Main and W3(OH). A comparison with carbon monoxide emission maps indicates
strong velocity gradients in the gas clumps hosting W3-Main and W3(OH) and show
small receding clumps of gas at IC 1795, suggestive of rapid gas removal
(faster than the T Tauri timescale) in the cluster forming regions. We discuss
one possible scenario for the progression of cluster formation in the W3
complex. We propose that early processes of gas collapse in the main structure
of the complex could have defined the progression of cluster formation across
the complex with relatively small age differences from one group to another.
However, triggering effects could act as catalysts for enhanced efficiency of
formation at a local level, in agreement with previous studies.
