Valency is the number of electrons it requires to achieve a stable octet configuration. If the last shell contains more than 4 electrons, it gains an electron, otherwise, it loses an electron due to stability.

Valency can be calculated from electronic configurations. 

In the given compound $\mathrm{Al} \left({\mathrm{PO}}_4\right)$, the metal ion is the aluminum ion $\left({\mathrm{Al}}^{3+}\right)$ and the valency of Al is 3. Now, as the compound is a neutral compound, the charge carried by the cation is equal to the charge carried by the anion.

The anion in the compound is a phosphate ion and thus, the anion is ${\mathrm{PO}}_4^{3-}$. Therefore, the valency of phosphate ion is 3.

Hence, the correct option is (C).