  Detection of higher order modes of gravitational waves in third-generation
ground-based detectors such as Cosmic Explorer and Einstein Telescope is
explored. Assuming an astrophysical population of binary black holes based on
events reported in the second gravitational wave catalog by LIGO and Virgo
(GWTC-2), we assess the detectability of these higher order modes using a
network consisting of three third-generation detectors. We find that the two
sub-leading modes ((3,3) and (4,4)) can be detected in around 80\% of the
population with a network signal-to-noise ratio of 10 or more, and for nearly
20\% of the sources, all six leading modes will be detectable. Besides, a study
concerning the effect of binary's mass ratio and its orbital inclination with
the observer's line-of-sight in detecting various modes is presented. For a few
selected events of the LIGO-Virgo catalog, we identify the modes that would
have been detected if a third-generation detector was operational when these
events were recorded. Observing these higher order modes in the 3G era would
have a huge impact on the science possible with these detectors ranging from
astrophysics and cosmology to testing strong-field gravity.
