7
H2O      solvent    0
H+       dissolved  1
OH-      dissolved  -1

H3PO4    dissolved  0
H2PO4-   dissolved  -1
HPO4--   dissolved  -2
PO4---   dissolved  -3

4
3 : 1 H+    1 OH-    -1 H2O       K 1.1e-14

3 : 1 H+    1 H2PO4-     -1 H3PO4        K 6.22e-3
3 : 1 H+    1 HPO4--     -1 H2PO4-       K 6.18e-8
3 : 1 H+    1 PO4---     -1 HPO4--       K 6.84e-13