Pacquiao vs Marquez Barrera vs Morales Marquez vs Vazquez Are a few where the 3rd fight was necessary.
Matt Skelton and Michael Sprott, if you count that their third fight was in Prizefighter. The second fight in 2007 nearly put me off boxing forever.
The ironic thing was that Minter had lost three out of four before that one and was heading nowhere. After the DQ he went on a run that landed the world title.