Catch a Gastly in 'Bellsprout Tower' train it untill it evolves into Haunter. Then train Haunter until He/She learns these attacks; Mean Look, Hypnosis, Lick & Night Shade. Now when you encounter Entei, Suicune and/or Raikou, no matter what level your haunter is always use Hypnosis, then Mean look. Then weaken it until it is almost knocked out. Put it to sleep again (unless it's already asleep) go mad throwing Ultraballs, Greatballs and any balls you have in your pack until you catch it. When the ball appears after you have thrown it Hold B+Up. keep doing this until you cat it/them.