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Why can phosphoric acid form three different series of phosphate salts?

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The structure of phosphorous acid, H3PO3. — oxyacid The structure of phosphorous acid, H₃PO₃.

Phosphoric acid (also known as orthophosphoric acid, $H_{3} P O_{4}$ ) can form three different series of phosphate salts because it is a triprotic acid. This means that in an aqueous solution, the molecule is capable of donating all three of its hydrogen atoms as protons.1

The three series of salts correspond to the replacement of one, two, or three of these hydrogen atoms:2

Dihydrogen phosphates: Contain the $H_{2} P O_{4}^{-}$ ion, formed by the replacement of one hydrogen atom.1
Hydrogen phosphates: Contain the $H P O_{4}^{2 -}$ ion, formed by the replacement of two hydrogen atoms.1
Orthophosphates: Contain the $P O_{4}^{3 -}$ ion, formed by the replacement of all three hydrogen atoms.1

Because each step of this deprotonation process involves different chemical properties, these salts exhibit varying levels of acidity or basicity when dissolved in water.1