How To Get Entei/Suicune/Raikou in Pokémon FireRed/LeafGreen Version
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Note: You must become Pokémon League Champion before finding the Legendary 3 Dogs.
You must catch at least 60 Pokémon in order for Professor Oak to upgrade your Pokédex to the National Pokédex.
You must complete the Sevii Islands to retrieve the Ruby & Sapphire.
You must defeat Team Rocket after the End Credits. (You'll see them at Mt. Ember)
If you chose Bulbasaur, you will find Entei.
If you chose Charmander, you will find Suicune.
If you chose Squirtle, you will find Raikou. (Like I did)
Alright, I was EXTREMELY LUCKY with this find, the legendary dogs travel each and every route once you enter and exit an area. Be sure to have a Pokémon under Level 50 so the Repel won't work for it to appear. Don't expect a quick find!! I was really lucky here:)
(I would advise you use the Master Ball for the 3 Legendary Dogs, they flee on the first turn).