Basicity of ${\mathrm{H}}_3{\mathrm{BO}}_3$ is

• It is a weak monobasic acid and in aqueous solution the boron atom completes its octet by removing OH- from water molecules

It, therefore, functions as a Lewis acid and not as a proton donor.
It behaves as strong acid when a polyhydroxy compound such as glycol or glycerol is added to its aqueous solution.