There are a total of 32 valence electrons for the H3PO4 Lewis structure. When we have an H (or H2 or H3) in front of a polyatomic molecule (like CO3, SO4, NO2, PO4,etc.) we know that it's an acid. This means that the Hydrogen atoms will be attached to the outside of the oxygen molecules
I do not know if you understand it but this is what my science book said.