The stable structures and aromatic characters for three cationic X 3 + (X = Sc, Y, and La) and three relevant neutral X3Cl (X = Sc, Y, La) clusters are investigated at the DFT and post HF level of theory. The calculated results show that the X 3 + cations each has two stable structures: the regular trigon (D3h) and the line ( $$ {{\hbox{D}}_{\infty {\rm{h}}}} $$ ) with the regular trigon (D3h) being the ground state, while for three neutral X3Cl clusters, Sc3Cl has three stable isomers: the trigon-pyramidal (C3v), bidentate (C2v-1), and C2v-2 structures, Y3Cl and La3Cl each has only two stable isomers: the trigon-pyramidal (C3v) and bidentate (C2v-1) structures. The ground states for three X3Cl species are all the bidentate (C2v-1) isomers. The calculations of the resonance energy (RE) and NICS show that trigonal X 3 + isomers exhibit higher degree of aromaticity. The detailed molecular orbital analyzes reveal that the isolated trigonal Sc 3 + and Y 3 + cations each has one delocalized π-type MO and shows single π-aromaticity, while the isolated trigonal La 3 + cation has one delocalized σ-type MO and shows single σ-aromaticity. The single π- or σ-aromaticity for X 3 + are attributed to the contributions mainly from the d AOs of the corresponding transition metal X atoms. However, when a singly negatively charged counterion Cl- is added to Sc 3 + , Y 3 + , and La 3 + cations respectively, the aromatic type for the two Sc 3 + , Y 3 + units in the corresponding neutral Sc3Cl, Y3Cl complexes are changed from π-aromaticity into σ-aromaticity, whereas the σ-aromaticity of the La 3 + units in the La3Cl complex keeps unchanged in this process. Thus three Sc 3 + , Y 3 + , La 3 + units in the corresponding X3Cl complexes all have only one σ-type MO and exhibit single σ-aromaticity.