Group 13-15 inorganic analogues of benzene, [HMYH]₃ (M = B, Al, Ga; Y = N, P, As), mixed heterocycles of the type [BAIGaNPAs]H₆ and their dimers have been theoretically examined at the B3LYP/TZVP level of theory. Six different isomers have been structurally characterized for the mixed compounds [BAIGaNPAs]H₆. B-N bonding strongly (about ∼90-100 kJ mol^-1) stabilizes the mixed heterocycles, followed by the preference of the Al-N bonded structures over Ga-N bonded (∼30-40 kJ mol^-1), while B-P bonding is slightly (5-10 kJ mol^-1) more favorable compared to B-As. Thus, the B-N-Al-As-Ga-P bonding pattern is predicted to be the most stable, followed by the B-N-Al-P-Ga-As core. Processes of [HMYH]₃ formation from donor-acceptor complexes H₃MYH₃ are predicted to be thermodynamically favorable for all MY combinations. Dimerization reactions of the coordinationally unsaturated [HMYH]₃ heterocycles yielding hexamer clusters [HMYH]₆ are found to be exothermic, with the exception of borazine, for which, as for benzene, dimerization is strongly endothermic due to the aromaticity of C₆H₆ and [HBNH]₃. Despite the high endothermicity of [HBNH]₃ dimerization, the B-N bond formation is the driving force of the dimerization of mixed species [BAIGaNPAs]H₆. The dimerization enthalpies of [BAIGaNPAs]H₆ may be both exo- and endothermic, depending on the bonding pattern of the isomers. A complete set of mean MY bond energies in four- and six-membered cycles of [HMYH]₆ was derived. The MY energies were found to be transferable quantities and may serve for a qualitative prediction of the relative stability of different isomers of mixed cluster compounds. [BAIGaNPAs]₂H₁₂ clusters are promising synthetic targets, they are expected to serve as single-source precursors for the stoichiometry-controlled CVD processes of the group 13-15 composites. A strategy of their synthesis and the most suitable starting systems have been also predicted.