Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for $H\to \text{invisible}$ decays where $H$ is produced according to the standard model via vector boson fusion, $Z(\ell \ell )H$, and $W/Z\left(\mathrm{had}\right)H$, all performed with the ATLAS detector using $36.1{\mathrm{fb}}^{-1}$ of $pp$ collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ at the LHC. In combination with the results at $\sqrt{s}=7$ and 8 TeV, an exclusion limit on the $H\to \text{invisible}$ branching ratio of $0.26\left(0.17_{-0.05}^{+0.07}\right)$ at 95% confidence level is observed (expected).