Absolute-cross-section measurements for electron-impact ionization of triply charged inert-gas ions:<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Ne</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Ar</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Kr</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>, and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Xe</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>

Electron-ion crossed beams have been employed to obtain absolute cross sections for single ionization of ${\mathrm{Ne}}^{3+}$, ${\mathrm{Ar}}^{3+}$, ${\mathrm{Kr}}^{3+}$, and ${\mathrm{Xe}}^{3+}$. These data reveal a steady progression from excellent agreement with theory and semiempirical formulas for ${\mathrm{Ne}}^{3+}$ to significant departures from any simple models for ${\mathrm{Xe}}^{3+}$. For ${\mathrm{Ar}}^{3+}$ some small departure from the models is apparent and the measurements agree with previous experimental results of M\"uller et al. For ${\mathrm{Kr}}^{3+}$ and ${\mathrm{Xe}}^{3+}$ the measured cross sections in the energy range below four threshold units exhibit complex structure and exceed estimates of direct ionization by roughly factors of 2 and 3, respectively. These features are attributed to indirect ionization processes involving excitation of inner-shell electrons, and the observed cross-section shapes imply that the excitation-autoionization features cannot be simply attributed to dipole-allowed excitation transitions. Ionization rates for Maxwellian electron-temperature distributions are calculated from the data.