Pure and Er-doped $({\left[\mathrm{Er}\right]}_{\text{crystal}}⩽2.4\times {10}^{20}{\mathrm{cm}}^{-3})$ , $\mathrm{Na}\mathrm{Bi}{\left(\mathrm{W}{\mathrm{O}}_4\right)}_2$ (NBW), $\mathrm{Na}\mathrm{Bi}{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_2$ , and $\mathrm{Li}\mathrm{Bi}{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_2$ crystals have been grown by the Czochralski method. The three crystal hosts have similar structural and optical properties. The noncentrosymmetric space group $I\overline{4}$ (No. 82) crystallographic structure has been established through $300\mathrm{K}$ single-crystal x-ray and neutron (for NBW only) diffraction. ${\mathrm{Er}}^{3+}$ energy levels were determined experimentally and simulated in the $S_4$ symmetry through a crystal-field analysis. With this background, the large spectroscopic bandwidths observed were ascribed to the presence of two $2b$ and $2d$ sites for ${\mathrm{Er}}^{3+}$ and to different short-range ${\mathrm{Na}}^{+}$ and ${\mathrm{Bi}}^{3+}$ distributions around both sites. The radiative properties of ${\mathrm{Er}}^{3+}$ are described by the Judd–Ofelt theory achieving branching ratios and radiative lifetimes for transitions useful as laser channels. The ${}^{4}I_{13∕2}\to {}^{4}I_{15∕2}$ laser channel $(\lambda \approx 1.5\mu \mathrm{m})$ shows a peak emission cross-section ${\sigma }_{\mathrm{EMI}}$ $(\lambda \approx 1530\mathrm{nm})\approx 0.5\times {10}^{-20}{\mathrm{cm}}^2$ and a quantum efficiency $\eta \approx 0.68$ to 0.74. The laser emission is envisaged to be tunable by $\Delta \lambda \approx 100\mathrm{nm}$ .