Iron phosphate when in acidic solution would dissociate into ions namely the iron ions and the phosphate ions. Furthermore, the phosphate ion would react to the hydronium ions forming HPO4^2-. To determine the net ionic equation we do as follows:
FePO4 <---------> Fe3+ + (PO4)3-
(PO4)3- + H3O+ <--------------> HPO4^2- + H2O
Adding the two equations would yield to:
FePO4(s) + H3O+(aq) ⇌ Fe^3+(aq) + HPO4^2−(aq) + H2O(l)