Electron-impact excitation of diatomic hydride cations I: HeH${}^{+}$, CH${}^{+}$, ArH${}^{+}$

{\bf R}-matrix calculations combined with the adiabatic nuclei approximation are used to compute electron-impact rotiational excitation rates for three closed-shell diatomic cations, HeH${}^{+}$, CH${}^{+}$, ArH${}^{+}$. Comparisons with previous studies show that an improved treatment of threshold effects leads to significant changes in the low temperature rates, furthermore the new calculations suggest that excitation of CH${}^{+}$ is dominated by $\mathrm{\Delta }J=1$ transitions as is expected for cations with a large dipole moment. A model for ArH${}^{+}$ excitation in the Crab Nebula is presented which gives results consistent with the observations for electron densities in the range $2-3\times 10^3$cm$^{-3}$.