Background and Objectives: Gas sensitivity of single semiconductor nanowires is strongly controlled by surface states and by the interplay between surface charge and bulk doping level. Nanowires based on III–V compounds such as GaAs, InAs and GaP combine high carrier mobility, controllable doping and large surface-to-volume ratio, which makes them promising building blocks for compact gas sensors. However, for these materials there is still no systematic quantitative description that would link nanowire diameter, doping type and level, and surface state modification by gas to the overall sensor response and resistance. The objective of this work is to develop a unified numerical model and to identify optimal operation regimes for gas sensors based on single GaAs, InAs and GaP nanowires at room temperature. Materials and Methods: We use a two-dimensional axisymmetric drift-diffusion-Poisson model implemented in COMSOL Multiphysics (Semiconductor module) to describe a single nanowire with diameter from 50 to 200 nm and length of 1 µm. The model explicitly accounts for surface states on the lateral nanowire surface and includes n- and p-type bulk doping over a wide range of concentrations. The action of the gas environment is introduced via a dimensionless parameter that describes the relative change in the surface state concentration and, consequently, in the surface charge density. For each combination of material, conductivity type, diameter and gas parameter we calculate the spatial distributions of potential and carrier concentrations and evaluate the nanowire conductance and resistance. The relative gas sensitivity is defined as the normalized change of conductance between the initial and gas–exposed states. Results: The calculated maps of gas sensitivity and resistance as functions of doping level, diameter and gas parameter reveal parameter regions where the nanowire conductance is strongly controlled by surface charge, while the resistance remains within a convenient range for electrical measurements. For GaAs nanowires with both n- and p-type conductivity, the model predicts a pronounced relative gas response in doping ranges where the resistance remains close to the upper boundary acceptable for single-nanowire readout. InAs nanowires provide somewhat lower maximal sensitivity but significantly lower resistance and wider sets of workable regimes due to the presence of an inversion surface accumulation layer. GaP nanowires show a narrower window of parameters where gas sensitivity and resistance are simultaneously suitable for practical applications. Conclusion: The developed model provides a unified framework for analyzing and comparing gas sensitivity of GaAs, InAs and GaP nanowires with different diameters and doping levels. The obtained maps of gas sensitivity and resistance can be used to select optimal combinations of material, diameter and doping for single-nanowire gas sensors and for hybrid sensing architectures where III–V nanowires are combined with other conducting nanomaterials.