The third metal ion, present in some DNA polymerases, does not have a well understood function. It is likely that, as methods to study mechanisms develop, the function of this transient third metal ion will become apparent. It has been postulated by some scientists that the third metal ion has different functions in different polymerases. This is an area of active research.
PPi stabilisation is done in polymerases with 2 metal ions, so is not dependent on the presence of a third metal ion (at least based on the current understanding).