To find the basicity of the given chemical compound first let us know what the term basicity means.
Basicity means a number of replaceable hydrogen atoms. Basicity of the molecule is the number of acidic hydrogen atoms. To be acidic, hydrogen must be bound to a highly electronegative atom such as oxygen or fluorine.
H3PO3 is called orthophosphorous acid or phosphorous acid. It is one of the phosphorus oxygenic acids. Phosphorous acid (H3PO3) forms salts known as phosphites which are sometimes used as reduction agents.
H3PO4→ 3H+ + PO4–
For hydrogen to be acidic it must be attached to a strongly electronegative atom. Here it is attached to 3 oxygen Hence basicity of H3PO4 is 3.