Beldum also has a catch rate of 3, and knows take down, and you can't actually just press A to activate the battle. Harder than any legendary imo. Took me ages to catch one.
The best tip is to hunt by night and use duskballs, or just have it already weakened (and paralysed/asleep) and let it run away thne everytime you encounter it throw a quickball on the first turn. Both good strategies for runners. I have caught every available legendary in both my Diamond and Platinum and still have my Masterball on both games.
On Ruby I
did manage to catch Rayquaza in a regular Pokéball while he was asleep and at full health. Haven't been able to emulate it since though, and I've tried.
Also, for the more mathematically minded of you, check this out if you haven't already:
http://bulbapedia.bulbagarden.net/wiki/Catch_rate
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