If you add a strong acid to a solution of phosphate buffer

Question

If you add a strong acid to a solution of phosphate buffer, which form(s) of phosphate in the equilibrium above would increase in abundance first? Which would decrease first? If you kept adding strong acid until the solution was no longer able to buffer, which form of phosphate would be present?

H₃PO₄ ↔ H⁺+H₂PO₄^2- ↔ 3H⁺+PO₄^3-