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The most luminous quasars at $z > 6$ are suspected to be both highlyclustered and reside in the most massive dark matter halos in the earlyUniverse, making them prime targets to search for galaxy overdensities and/orprotoclusters. We search for Lyman-break dropout-selected galaxies using HSTWFC3/ACS broadband imaging in the fields of three $6 < z < 7$ quasars, as wellas their simultaneously observed coordinated-parallel fields, and constraintheir photometric redshifts using EAZY. One field, J0305-3150, shows a volumedensity 10$\times$ higher than the blank-field UV luminosity function (UVLF) atM$_{UV} < -20$, with tentative evidence of a 3$\sigma$ overdensity in itsparallel field located 15 cMpc away. Another field, J2054-0005, shows anangular overdensity within 500 ckpc from the quasar but still consistent withUVLF predictions within 3$\sigma$, while the last field, J2348-3054, shows noenhancement. We discuss methods for reducing uncertainty in overdensitymeasurements when using photometric selection and show that we can robustlyselect LBGs consistent with being physically associated with the quasar,corroborated by existing JWST/NIRCam WFSS data in the J0305 field. Evenaccounting for incompleteness, the overdensities in J0305 and J2054 are higherfor brighter galaxies at short angular separations, suggesting preferentialenhancement of more massive galaxies in the immediate vicinity of the quasar.Finally, we compare the LBG population with previously-identified [CII] andmm-continuum companions; the LBG overdensities are not accompanied by anenhanced number of dusty galaxies, suggesting that the overdense quasar fieldsare not in the bursty star-forming phase sometimes seen in high-redshiftprotoclusters.

A Mixture of LBG Overdensities in the Fields of Three $6 < z < 7$ Quasars: Implications for the Robustness of Photometric Selection