  We quantify galaxy overdensities around three high-redshift quasars with
known [CII] 158um companions: PJ231-20 (z=6.59), PJ308-21 (z=6.24) and
J0305-3150 (z=6.61). Recent SCUBA2 imaging revealed the presence of 17
submillimeter galaxies (SMG) with sky separations $0.7'< \theta < 2.4'$ from
these three quasars. We present ALMA Band 6 follow-up observations of these
SCUBA2-selected SMGs to confirm their nature and redshift. We also search for
continuum-undetected [CII] emitters in the ALMA pointings and make use of
archival MUSE observations to search for Lyman-$\alpha$ Emitters (LAE)
associated with the quasars. While most of the SCUBA2-selected sources are
detected with ALMA in the continuum, no [CII] line emission could be detected,
indicating that they are not at the quasar redshifts. Based on the
serendipitous detection of CO 7-6 and [CI] emission lines, we find that four
SMGs in the field of PJ231-20 are at z~2.4, which is coincident with the
redshift of a Mg II absorber in the quasar rest-frame UV spectrum. We report
the discovery of 2 LAEs within <0.6 cMpc of PJ231-20 at the same redshift,
indicating a LAE overdensity around this quasar. Taken together, these
observations provide new constraints on the large-scale excess of
Lyman-$\alpha$- and [CII]-emitting galaxies around z>6 quasars and suggest that
only wide-field observations, such as MUSE, ALMA or JWST mosaics, can reveal a
comprehensive picture of large-scale structure around quasars in the first
billion years of the Universe.
