First, you need to break into Team Galactic's giant warehouse in Veilstone and once you finally make it up to the boss, he'll give you a master ball if you defeat him.
You have to go to the lottery corner. The lady will ask you to draw and if you get the same number as the ticket you will get a master ball. You can do it once a day.
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Get rid of some of the master balls! the number of master balls you have is the number of the pokemon
There's no such guy in Sandgem Town that has 3 Master Balls.
There is only one master ball in Pokemon diamond. But there is a cheat for getting a WHOLE lot more. Look it up at how do you get 98 master balls? I don't know where the master ball is I think it's in a team galatic headquarters.
action replay is best
you get the master ball once you beat Cyrus in the galactic building