PO3 is not a complete chemical formula. It's missing an overall charge to represent an ion. It's likely a typo and should be either PO₃³⁻ (phosphate ion) or PO₃⁻¹ (phosphite ion).

Let me explain:

* PO₄³⁻ is the phosphate ion, which has a -3 charge. It consists of one phosphorus atom (P) and four oxygen atoms (O).

* PO₃³⁻ is the phosphite ion, which has a -3 charge. It consists of one phosphorus atom (P) and three oxygen atoms (O).

Therefore, the correct ionic charge for PO₃ depends on the specific chemical formula you are referencing.