If multiple spells with different names are prepped to Reth's Unbound III breach using Link or a different method ("Any number of spells with the same name may be prepped to an opened Unbound breach."), then no additional spells can be prepped using the breach's special rules.
Link spells do not override the special rules of the Unbound breach. Any number of the same spell with Link can be prepped to the opened Unbound breach (not just two).
