What is the equilibrium associated with ka3 for h3po4?

Phosphoric acid has 3 pKa values (pKa1=2.1, pKa2=6.9, pKa3 = 12.4) and after 3 ionization it gives 3 types of ions at different pKa values:

H₃PO₄ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + H₂PO₄⁻ (aq) pKₐ₁

H₂PO₄⁻ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + HPO₄²⁻ (aq) pKₐ₂

HPO₄²⁻ (aq) + H₂O (l) ⇌ H₃O⁺ (aq) + PO₄³⁻ (aq) pKₐ₃

The last equilibrium is associated with the highest pKa value (12.4) of phosphoric acid. There the last OH group will lose its hydrogen and hydrogen phosphate ion (HPO₄²⁻) turns into phosphate ion (PO₄³⁻).