The structural and thermodynamic properties of oligomeric anions [M _n X_{3n+ 1}]^- (M = Al, Ga, In; X = F, Cl, Br, I; n = 2, 3, 4) have been obtained by the density functional theory B3LYP method with the LAN2DZ(d) and LAN2DZ(d)+ basis sets. A wide diversity of structural isomers was found for trimeric fluoride anions M₃F ₁₀ ^- . Among the trimers, except In₃F ₁₀ ^- , the most stable is a linear isomer composed of two MX₃ molecules coordinated to the MX ₄ ^- anion. The formation of tetrameric anions M₄X ₁₃ ^- was demonstrated to be thermodynamically allowed at low temperatures at MX₃: X ^- > 4: 1. The existence of higher oligomers is less probable. The affinity of oligomer halides (MX₃) _n for halide ions increases with an increase in n. The propensity to form oligomeric anions decreases in the series F > Cl = Br > I. The fluoride systems show a tendency to form structures with CN = 5 and 6, these structures for In being the most stable.