  We present a comprehensive analysis of 653 optical candidate counterparts
reported during the third gravitational wave (GW) observing run. Our sample
concentrates on candidates from the 15 events (published in GWTC-2, GWTC-3 or
not retracted on GraceDB) that had a >1% chance of including a neutron star in
order to assess their viability as true kilonovae. In particular, we leverage
tools available in real time, including pre-merger detections and
cross-matching with catalogs (i.e. point source, variable star, quasar and host
galaxy redshift datasets), to eliminate 65% of candidates in our sample. We
further employ spectroscopic classifications, late-time detections and light
curve behavior analyses, and conclude that 66 candidates remain viable
kilonovae. These candidates lack sufficient information to determine their
classifications, and the majority would require luminosities greater than that
of AT2017gfo. Pre-merger detections in public photometric survey data and
comparison of catalogued host galaxy redshifts with the GW event distances are
critical to incorporate into vetting procedures, as these tools eliminated >20%
and >30% of candidates, respectively. We expect that such tools which leverage
archival information will significantly reduce the strain on spectroscopic and
photometric follow-up resources in future observing runs. Finally, we discuss
the critical role prompt updates from GW astronomers to the EM community play
in reducing the number of candidates requiring vetting.
