how do you write the ionization reaction and the acid ionization constant expression for the third ionization of phosphoric acid in water? Can you explain the main concepts that are involved in this?

Answer 1

Just do them one at a time.
H3PO4 ==> H^+ + H2PO4^-
H2PO4^- ==> H^+ + HPO4^2-
HPO4^- ==> H^+ + PO4^3-

K is ALWAYS (products)/(reactants) and ate coefficients become exponents.
k3 = (H^+)(PO4^3-)/(HPO4^-)
The coefficients are 1 so everything is raised to the 1st power.