The structures of compressed halogen polyhydrides HnX (X = F, Cl and n > 1) and their evolution under pressure are studied using ab initio calculation based on density functional theory. HnF (n > 1) are metastable up to 300 GPa, whereas for HnCl (n > 1), four new stoichiometries (H2Cl, H3Cl, H5Cl, and H7Cl) are predicted to be stable at high pressures. Interestingly, triangular H-3(+) species are unexpectedly found in stoichiometries H2F with [H-3] [HF2](-), H-3(+) with [113] [F]-, H5F with [H-3](+)[HF2](-)[H-2](3), and H5Cl with [H-3](+)[Cl](-)[H-2] above 100 GPa. Importantly, formation processes of H-3(+) species are clearly seen on the basis of comparing bond lengths, bond overlap populations, electron localization functions, and Bader charges as a functions of pressure. Further analysis reveals that the formation of H-3(+) species is attributed to the pressure-induced charge transfer from hydrogen atoms to halogen atoms.