  A search for supersymmetry involving the pair production of gluinos decaying
via third-generation squarks into the lightest neutralino
($\displaystyle\tilde\chi^0_1$) is reported. It uses LHC proton-proton
collision data at a centre-of-mass energy $\sqrt{s} = 13$ TeV with an
integrated luminosity of 36.1 fb$^{-1}$ collected with the ATLAS detector in
2015 and 2016. The search is performed in events containing large missing
transverse momentum and several energetic jets, at least three of which must be
identified as originating from $b$-quarks. To increase the sensitivity, the
sample is divided into subsamples based on the presence or absence of electrons
or muons. No excess is found above the predicted background. For
$\displaystyle\tilde\chi^0_1$ masses below approximately 300 GeV, gluino masses
of less than 1.97 (1.92) TeV are excluded at 95% confidence level in simplified
models involving the pair production of gluinos that decay via top (bottom)
squarks. An interpretation of the limits in terms of the branching ratios of
the gluinos into third-generation squarks is also provided. These results
improve upon the exclusion limits obtained with the 3.2 fb$^{-1}$ of data
collected in 2015.
