  GRANDMA is a network of 25 telescopes of different sizes, including both
photometric and spectroscopic facilities. The network aims to coordinate
follow-up observations of gravitational-wave candidate alerts, especially those
with large localisation uncertainties, to reduce the delay between the initial
detection and the optical confirmation. In this paper, we detail GRANDMA's
observational performance during Advanced LIGO/Advanced Virgo Observing Run 3
(O3), focusing on the second part of O3; this includes summary statistics
pertaining to coverage and possible astrophysical origin of the candidates. To
do so, we quantify our observation efficiency in terms of delay between
gravitational-wave candidate trigger time, observations, and the total
coverage. Using an optimised and robust coordination system, GRANDMA
followed-up about 90 % of the gravitational-wave candidate alerts, i.e. 49 out
of 56 candidates. This led to coverage of over 9000 deg2 during O3. The delay
between the gravitational-wave candidate trigger and the first observation was
below 1.5 hour for 50 % of the alerts. We did not detect any electromagnetic
counterparts to the gravitational-wave candidates during O3, likely due to the
very large localisation areas (on average thousands of degrees squares) and
relatively large distance of the candidates (above 200 Mpc for 60 % of BNS
candidates). We derive constraints on potential kilonova properties for two
potential binary neutron star coalescences (GW190425 and S200213t), assuming
that the events' locations were imaged.
