H3BO3 is
Monobasic and weak Lewis acid
H3BO3 is better written as B(OH)3 where the central Boron atom is electron deficient with only 6 valence electrons. In its reaction with water, it accepts an OH− thus releasing H+ from water.
H3BO3+H2O⇌[B(OH)4]−+H+
Each of the three oxygen atoms in the structure has 2 lone pairs (each). These lone pairs try and reduce the electron deficiency in Boron atom via back bonding. The oxygen – boron bonds tend to have a slight double bond character. Hence, the acid – boric acid – is monobasic and a weak Lewis acid.