Answer
See answer below.
Work Step by Step
First hydronium:
$H_3PO_4(aq)+H_2O(l)\rightarrow H_2PO_4^-(aq)+H_3O^+(aq)$
Second hydronium:
$H_2PO_4^-(aq)+H_2O(l)\rightarrow HPO_4^{2-}(aq)+H_3O^+(aq)$
Third hydronium:
$HPO_4^{2-}(aq)+H_2O(l)\rightarrow PO_4^{3-}(aq)+H_3O^+(aq)$
