Limits are set on the pair production of scalar leptoquarks, where all possible decays of the leptoquark into a quark ($t$, $b$) and a lepton ($\tau$, $\nu$) of the third generation are considered. The limits are presented as a function of the leptoquark mass and the branching ratio into charged leptons for up-type ($\mathrm{LQ}3u$ $\to t\nu /b\tau$) and down-type ($\mathrm{LQ}3d$ $\to b\nu /t\tau$) leptoquarks. Many results are reinterpretations of previously published ATLAS searches. In all cases, LHC proton-proton collision data at a centre-of-mass energy of $s$ = 13 TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. Masses below 800 GeV are excluded for both $\mathrm{LQ}3u$ and $\mathrm{LQ}3d$ independently of the branching ratio, with masses below about 1 TeV being excluded for the limiting cases of branching ratios equal to zero or unity.