  With the conclusion of the third observing run for Advanced LIGO/Virgo (O3),
we present a detailed analysis of both triggered and serendipitous observations
of 17 gravitational wave (GW) events (7 triggered and 10 purely serendipitous)
from the Searches After Gravitational-waves Using ARizona Observatories
(SAGUARO) program. We searched a total of 4935 deg$^2$ down to a median
5$\sigma$ transient detection depth of 21.1 AB mag using the Mt Lemmon 1.5 m
telescope, the discovery engine for SAGUARO. In addition to triggered events
within 24~hours, our transient search encompassed a time interval following GW
events of $<120$~hrs, providing observations on $\sim$ 1/2 of the events
accessible to the Mt Lemmon 1.5 m telescope. We covered 2.1--86\% of the LVC
total probability ($P_{\rm total}$) for individual events, with a median
$P_{\rm total} \approx 8\%$ within $<120$~hours. Following improvements to our
pipeline and the addition of serendipitous observations, we find a total of 7
new optical candidates across 5 GW events which we are unable to rule out after
searching for additional information and comparing to kilonova models. Using
both publicly available and our own late-time data, we investigated a total of
252 optical candidates for these 17 events, finding only 65\% were followed up
in some capacity by the community. Of the total 252 candidates, we are able to
rule out an additional 12 previously reported counterpart candidates. In light
of these results, we discuss lessons learned from the SAGUARO GW counterpart
search. We discuss how community coordination of observations and candidate
follow-up, as well as the role of archival data, are crucial to improving the
efficiency of follow-up efforts and preventing unnecessary duplication of
effort with limited EM resources.
