Defect structures and site-preferences of tenary elements in an off-stoichiometric transition metal Al3Nb-based trialuminides are predicted by the pseudo-ground state analysis based on the first nearest neighbor approximation. The calculated stable defect structures are structure-type (vacancy type) defect in Al-rich side and antistructure-type (substitution type) defect in Nb-rich side of stoichiometry. Additional elements that belong to VII a through V b group in the periodic table and Be are Al-site substitutions, and those that belong to I a through IV a group except for Be are Nb-site substitutions in any compositions. Moreover, a guide to alloy design of Al3Nb is also discussed.