The structure of ${\mathrm{PO}}_4^{3-}$ is as follows

In the given molecule, The P atom have 5 valence electrons. The hybridization can be determined by the number of sigma bonds and the number of lone pair.

As phosphorous central atom forms four $\left(4\right)$ sigma bonds and has no lone pair of electrons. It uses four hybrid orbital and thus, hybridisation of $\mathrm{p}$-atom is ${\mathrm{sp}}^3$.