  We present a Herschel/SPIRE survey of three protoclusters at z=2-3
(2QZCluster, HS1700, SSA22). Based on the SPIRE colours (S350/S250 and
S500/S350) of 250 $\mu$m sources, we selected high redshift dusty star-forming
galaxies potentially associated with the protoclusters. In the 2QZCluster
field, we found a 4-sigma overdensity of six SPIRE sources around 4.5' (~2.2
Mpc) from a density peak of H$\alpha$ emitters at z=2.2. In the HS1700 field,
we found a 5-sigma overdensity of eight SPIRE sources around 2.1' (~1.0 Mpc)
from a density peak of LBGs at z=2.3. We did not find any significant
overdensities in SSA22 field, but we found three 500 $\mu$m sources are
concentrated 3' (~1.4 Mpc) east to the LAEs overdensity. If all the SPIRE
sources in these three overdensities are associated with protoclusters, the
inferred star-formation rate densities are 10$^3$-10$^4$ times higher than the
average value at the same redshifts. This suggests that dusty star-formation
activity could be very strongly enhanced in z~2-3 protoclusters. Further
observations are needed to confirm the redshifts of the SPIRE sources and to
investigate what processes enhance the dusty star-formation activity in z~2-3
protoclusters.
