Exact equi-atomic senary alloys including three elements from 3d, 4d and 5d transition metals (TMs), such as (Ti, Zr, Hf), (Cu, Ag, Au) and (Ni, Pd, Pt), were investigated for their forming into solid solutions as high-entropy alloys (HEAs). Three alloys of CuCoPdTiZrHf, FeCuCoTiZrHf and CuAgAuNiPdPt were principally investigated as candidates selected with a help of an alloy design utilizing Pettifor map for binary compounds with several stoichiometries and binary phase diagrams. Besides, an Al<sub>4</sub>CoNiPdPt as a marginal alloy was also investigated in a framework of this alloy design. The XRD analysis revealed that the CuCoPdTiZrHf was formed into bcc structure, whereas the FeCuCoTiZrHf and Al<sub>4</sub>CoNiPdPt alloys were CsCl and CuAgAuNiPdPt was fcc structure. It was found that the crystallographic structures of the CuCoPdTiZrHf, FeCuCoTiZrHf and CuAgAuNiPdPt HEAs as well as Al<sub>4</sub>CoNiPdPt alloy can be explained by valence electron concentration principally.