The way to find Rayku, Entei, or Suicune you must catch all the Unowns, A-Z and ! and ?. After you cought all 28 Unowns, you will find Rayku, Entei, or Suicune roaming around Kento. You could catch it with many tries with the Ultra Ball, but if you haven't already foolishly used your Master Ball, it would be wise to catch your legendary dog Pokemon with the Master Ball. It depends what your starter Pokemon what legendary dog Pokemon you will find.