I am uncertain about the structure of $\ce {H_3PO_4}$ because different sources show different ones.
The structure shown here is:
while Wikipedia gives this one:
Arguments in favour of the 2nd structure:
- Structure having filled octet for second row elements (C, N, O, F) are stable.
- Structure having minimum number of formal charges and maximum number of bonds.
- Structure in which there is minimal charge separation while keeping the formal charges closer together.
Arguments supporting the first structure:
- Structure in which negative charge appears on the most electronegative atom should be chosen.
- I feel that this could be more contributing because in the first structure sulphur has an expanded octet while $d$ orbitals are of higher energy so it's more favourable to form bonds without using them.
- Structure having filled octet a for second row elements ($\ce {C, N, O, F}$) are stable. (followed by both structures)
$\ce {H_3PO_4}$ is different from phosphate ion.None of the answers in the linked question directly state what would be the more contributing structure. They focus on the hybridisation of P in phosphate.