39From Wiki - "Even though all three hydrogen (H) atoms are equivalent on an orthophosphoric acid molecule, the successive Ka values differ since it is energetically less favorable to lose another H+ if one (or more) has already been lost and the molecule/ion is more negatively-charged." -> Hence the conjugate base successively becomes less stable as more protons are lost -> Hence acidity is not as much as in hypophosphorous acid which is monoprotic and loses one proton easily.
I wonder why the energy issues though, because resonance should make up for that...
