  We present the discovery of three protoclusters at $z\sim3\mathrm{-}4$ with
spectroscopic confirmation in the Canada-France-Hawaii Telescope (CFHT) Legacy
Survey Deep Fields. In these fields, we investigate the large-scale projected
sky distribution of $z\sim3\mathrm{-}6$ Lyman break galaxies and identify 21
protocluster candidates from regions that are overdense at more than $4\sigma$
overdensity significance. Based on cosmological simulations, it is expected
that more than $76\%$ of these candidates will evolve into a galaxy cluster of
at least a halo mass of $10^{14}\,\mathrm{M_\odot}$ at $z=0$. We perform
follow-up spectroscopy for eight of the candidates using Subaru/FOCAS,
KeckII/DEIMOS, and Gemini-N/GMOS. In total we target 462 dropout candidates and
obtain 138 spectroscopic redshifts. We confirm three real protoclusters at
$z=3\mathrm{-}4$ with more than five members spectroscopically identified, and
find one to be an incidental overdense region by mere chance alignment. The
other four candidate regions at $z\sim5\mathrm{-}6$ require more spectroscopic
follow-up in order to be conclusive. A $z=3.67$ protocluster, which has eleven
spectroscopically confirmed members, shows a remarkable core-like structure
composed of a central small region ($<0.5\,\mathrm{physical\>Mpc}$) and an
outskirts region ($\sim1.0\,\mathrm{physical\>Mpc}$). The Ly$\alpha$ equivalent
widths of members of the protocluster are significantly smaller than those of
field galaxies at the same redshift while there is no difference in the UV
luminosity distributions. These results imply that some environmental effects
start operating as early as at $z\sim4$ along with the growth of the
protocluster structure.
