The stability table and superconducting properties such as screened Coulomb potential, electron-phonon coupling and critical temperature of planar boron sheet included ternary crystalline compounds of MgB2-type M0.5Al0.5B2 (M = Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Y, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re, and Os) have been investigated by first-principles density functional theory calculations with spin-orbit coupling. It is shown that the M = Li, Mg, Ca, Sc, Ti, V, Y, Zr, Nb, Mo, Tc, Hf, Ta, and W-substituted compounds are thermodynamically stable at the ambient conditions. Among them, Nb0.5Al0.5B2 has the lowest cohesive energy while Ti0.5Al0.5B2 has the lowest formation enthalpy. Except for Be0.5Al0.5B2 , all compounds are mechanically stable, and fourteen of them are dynamically stable also. So, a dozen of M0.5Al0.5B2 (M = Li, Mg, Ca, Sc, Ti, V, Zr, Nb, Mo, Tc, Hf, and Ta) compounds are satisfying all three stability conditions. The calculated electronic properties show that all structures have metallic character and interestingly the radii of the substituted atoms correlate with surface area of regular B6 hexagons and volumes of metal-boron pyramids. All of them are hard materials, and V0.5Al0.5B2 has the highest semi-empirical microhardness (27 GPa). According to the precise phonon dispersion and electronphonon coupling calculations, T-c's of the stable compounds are lower than that of MgB2 (39 K), and Tc0.5Al0.5B2 has higher T-c (similar to 7 K) than the others.