We quantify galaxy overdensities around three high-redshift quasars with known [C ii ] _{158 μ m} 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 (SMGs) with sky separations 0.′7 < θ < 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 [C ii ] _{158 μ m} emitters in the ALMA pointings and make use of archival MUSE observations to search for Ly α emitters (LAEs) associated with the quasars. While most of the SCUBA2-selected sources are detected with ALMA in the continuum, no [C ii ] _{158 μ m} line emission could be detected, indicating that they are not at the quasar redshifts. Based on the serendipitous detection of CO 7–6 and [C i ] _{809 μ m} 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 an Mg ii absorber in the quasar rest-frame UV spectrum. We report the discovery of two LAEs within <0.6 cMpc of PJ231–20 at the same redshift, indicating an LAE overdensity around this quasar. Taken together, these observations provide new constraints on the large-scale excess of Ly α - and [C ii ] _{158 μ m} -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.