The density functional theory (DFT) methods were used for theoretical studies of three compounds. The guanidine molecule is a main structural unit in these complexes, while the studied acids have similar chemical character. The family of simple guanidinium compounds is intensively investigated as potential materials with ferroic phase transitions. Among studied "guanidinium" complexes, those with crystal structure without macroscopic center of inversion, exhibit NLO properties. For three compounds: CN(3)H(5)CH(3)COOH, CN(3)H(5)CCl(3)COOH and CN(3)H(5)CF(3)COOH the detailed theoretical calculations were performed. For each complex the equilibrium geometry was obtained. The calculated geometrical parameters (bond lengths and angles) of all investigated complexes are compared. The theoretical vibrational frequencies and potential energy distribution (PED) of three "guanidinium" compounds were calculated by B3LYP method. On the basis of PED calculations the detailed assignments of bands for new chemical complexes are presented. The real crystals were obtained in the case of CN(3)H(5)CH(3)COOH compound, only. The theoretical results can be used in future, when the synthesis of trichloroacetic and trifluoroacetic analogues of CN(3)H(5)CH(3)COOH will be done.