Sadly, there is no way to get them all three. Except by you playing the game 3 times and trading with another game like Leaf Green, Fire Red, Sapphire, Ruby and Emerald. It is an awful lot of work. But lets start with the basics, you get one (1) legendary beast without needing to re-play the game from scratch and deleting your progress, depending on your starter. If you chose Charmander as a starter you will get Suicune at the end of the game. If you chose Squirtle as a starter you will get Raikou at the end of the game. Why? Because he will run from you as soon as you encounter it.