Boric acid, H3BO3, in aqueous solution would only give out one H+ ion. As it is also produce OH ion and by hydrolysis it produces one proton. All the boron compounds (BX3) are having only 6 valence electrons in it and should follow the octet rule by taking another electron.
B(OH)3 + 2 H2O → B(OH)4− + H3O