H3PO4 it contains 3 replaceable Hydrogen atoms H3PO4 is a tribasic acid. It combines with alkalies like NaOH to form three series of salts.
H3PO4+NaOH→NaH2PO4SodiumDihydrogenphosphate+H2O
H3PO4+2NaOH→Na2HPO4Disodiumhydrogen+2H2O
H3PO4+3NaOH→Na3PO4SodiumPhosphate+3H2O.