  Using a semi-classical model, we investigate frustrated double ionization
(FDI) in $\mathrm{D_3^+}$, a two-electron triatomic molecule, when driven by an
intense, linearly polarized, near-infrared (800 nm) laser field. We compute the
kinetic energy release of the nuclei and find a good agreement between
experiment and our model. We explore the two pathways of FDI and show that,
with increasing field strength, over-the-barrier ionization overtakes tunnel
ionization as the underlying mechanism of FDI. Moreover, we compute the angular
distribution of the ion fragments for FDI and identify a feature that can
potentially be observed experimentally and is a signature of only one of the
two pathways of FDI.
