In the present report mixed cubic clusters [(HAl)_x(HGa)_y(HIn)_z(YH)₄] (Y=N, P, As, x+y+z=4) and [(HM)₄(NH)_k(PH)₁(AsH)_m] (M=Al, Ga, In, k+l+m=4) are theoretically investigated for the first time. Structural parameters, vibrational frequencies and thermodynamic properties have been obtained at B3LYP/LANL2DZ(d,p) level of theory. At high temperatures, mixed clusters are predicted to be thermodynamically more stable compared to mixture of homoatomic cubanes. Nitrogen-containing clusters have higher stability toward dissociation and are formed more exothermically compared to P, As analogs. Thus, mixed nitrogen clusters are expected to serve as advanced single-source precursors for CVD of mixed alloys, such as Al_xGa_yIn_1-x-yN. Variation of the Al:Ga:In ratio is possible by using higher oligomers [HMYH]_n (n=6-12).