  We report the discovery of an extremely dense group of massive galaxies at
the centre of the protocluster at $z=3.09$ in the SSA22 field from
near-infrared spectroscopy conducted with the Multi-Object InfraRed Camera and
Spectrograph (MOIRCS) equipped on the Subaru Telecope. The newly discovered
group comprises seven galaxies confirmed at $z_{\rm spec}\approx3.09$ within
180 kpc including five massive objects with the stellar masses larger than
$10^{10.5}~M_{\odot}$ and is associated with a bright sub-mm source
SSA22-AzTEC14. The dynamical mass of the group estimated from the line-of-sight
velocity dispersion of the members is $M_{\rm
dyn}\sim1.6\pm0.3\times10^{13}~M_{\odot}$. Such a dense group is expected to be
very rare at high redshift as we found only a few comparable systems in
large-volume cosmological simulations. Such rare groups in the simulations are
hosted in collapsed halos with $M_{\rm vir}=10^{13.4}-10^{14.0}~M_{\odot}$ and
evolve into the brightest cluster galaxies (BCGs) of the most massive clusters
at present. The observed AzTEC14 group at $z=3.09$ is therefore very likely to
be a proto-BCG in the multiple merger phase. The observed total stellar mass of
the group is $5.8^{+5.1}_{-2.0}\times10^{11}~M_{\odot}$. It suggests that over
half the stellar mass of its descendant had been formed by $z=3$. Moreover, we
identified over two members for each of the four Ly$\alpha$ blobs (LABs) using
our new spectroscopic data. This verifies our previous argument that many of
the LABs in the SSA22 protocluster associated with multiple developed stellar
components.
