All I can say is that you're getting unlucky. There have been many times where I throw any kind of Pokéball at a legendary and the ball wobbles 3 times. The legendary always escapes. Just keep trying and you'll eventually catch one.
Tips: -Use False Swipe or Hold Back to keep the Pokemon at 1 HP
-Status conditions help you catch the Pokemon.
-Use Ultra Balls or Timer Balls. Timer Balls are very useful later into a battle.