I need all 3
You need to soar with a lvl 100 Pokemon in your party. Fabled Cave is south east of mauville. You can only get either Zekrom or Reshiram depending on which game you have. The other must be traded over. Kyurem will appear at the Gnarled Den near dewford if you have both the others in your party.DNA splicer can be found by using the dowsing machine in the Gnarled Den.
Hope I Helped!