  W3 is one of the most outstanding regions of high-mass star formation in the
outer solar circle, including two active star-forming clouds, W3 Main and
W3(OH). Based on a new analysis of the $^{12}$CO data obtained at
38$^{\prime\prime}$ resolution, we have found three clouds having molecular
mass from 2000 to 8000~$M_\odot$ at velocities, $-50$~km s$^{-1}$, $-43$~km
s$^{-1}$, and $-39$~km s$^{-1}$. The $-43$~km s$^{-1}$ cloud is the most
massive one, overlapping with the $-39$~km s$^{-1}$ cloud and the $-50$~km
s$^{-1}$ cloud toward W3 Main and W3(OH), respectively. In W3 Main and W3(OH),
we have found typical signatures of a cloud-cloud collision, i.e., the
complementary distribution with/without a displacement between the two clouds
and/or a V-shape in the position-velocity diagram. We frame a hypothesis that a
cloud-cloud collision triggered the high-mass star formation in each region.
The collision in W3 Main involves the $-39$~km s$^{-1}$ cloud and the $-43$~km
s$^{-1}$ cloud. The collision likely produced a cavity in the $-43$~km s$^{-1}$
cloud having a size similar to the $-39$~km s$^{-1}$ cloud and triggered the
formation of young high-mass stars in IC~1795 2 Myr ago. We suggest that the
$-39$~km s$^{-1}$ cloud is still triggering the high-mass objects younger than
1 Myr embedded in W3 Main currently. On the other hand, another collision
between the $-50$~km s$^{-1}$ cloud and the $-43$~km s$^{-1}$ cloud likely
formed the heavily embedded objects in W3(OH) within $\sim$0.5 Myr ago. The
present results favour an idea that cloud-cloud collisions are common phenomena
not only in the inner solar circle but also in the outer solar circle, where
the number of reported cloud-cloud collisions is yet limited (Fukui et al.
2021, PASJ, 73, S1).
