We report the results of a theoretical study of MnNn (M = Al, Ga, and In; n = 4, 5, 6) neutral and anionic clusters, focusing on the changes in structural and electronic properties upon the addition of an electron to the corresponding neutral clusters. Overall, the extra electron did not induce significant structural changes in AlnNn clusters, whereas it affected significantly the lowest energy configurations of GanNn and InnNn clusters. This may be attributed to the dominance of N-N bonds in GanNn and InnNn clusters in contrast to the dominance of Al-N bonds in AlnNn clusters. The atomic charge analysis showed that the extra electron is localized on the metal atoms, irrespective of the lowest energy structural configurations of these clusters.