Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O-2) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M-3(+)) produced via the corresponding dimer radical cation (M-2(+)) is clearly seen in the solution containing O-2 as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M-3(+) CR absorption is observed; this is due to the large M-3(+)/M-2(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at similar to 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M-3(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M-3(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.