The weakly bound trimer between two argon atoms and a molecule of hydrogen bromide was identified from its rotational spectrum observed at conditions of supersonic expansion. The spectrum was measured with a cavity, Fourier transform microwave spectrometer and the rotational, quartic, and bromine hyperfine coupling constants were determined for the two most abundant isotopomers Ar-40(2) (HBr)-Br-79 and Ar-40(2) (HBr)-Br-81. The various spectroscopic observables dependent on the harmonic force field were successfully reproduced using ab initio force field calculations. Experimental moments of inertia were then used, together with calculated vibration-rotation inertial contributions to derive the r(z) geometry. The resulting d(ArBr) = 4.166 Angstrom, d(ArAr) = 3.840 Angstrom, compare with r(z) distances d(ArBr) = 4.155 Angstrom for ArHBr, and d(ArAr) = 3.836 Angstrom for Ar-2. Angular information derived from the measured nuclear hyperfine splitting constants and from the electric dipole moment was found to be in remarkable agreement. Comparisons of the available information on the interaction energy and geometries for the ArHX and the Ar2HX series of clusters have been made in order to assess the relationship between the experimental ground state based structural parameters and the desired equilibrium geometry.