How do you get Raikou / Entei / Suicine?

You can't without a cheat device.

You can only get Raikou/Suicune/Entei in FireRed or LeafGreen if choose a specific starter. I don't know which starter gets you Suicune, Entei or Raikou, but I'm sure somebody here knows it.

You will need to use a cheating device or trade with a friend.

You have to trade from Pokemon fire red and leaf green or you could use a cheat device action replay or game shark but if I were you I would just trade because cheating often skrews up you're game.

You could either trade with a friend, trade them from FireRed/LeafGreen, or use a cheat code.

Raikou , Entei and Suicune are obtainable in fire red and leaf green. You can get them depending on your starter.

In FireRed version or LeafGreen.
If you get Squirtle it's a Raikou you find.
Charmander- Suicune (I have one)
Bulbasuar- Entei
(You MUST beat the E4 and find it in the wild like the Lati's)
Hope I helped.

~OblivionDestiny~

You will need to trade from the v_2 games also called as version 2 also called as Pokemon Fire Red and Pokemon Leaf Green. You could also cheat with Action Replay or GameShark. If you feel like.

You can obtain them in Pokemon Colosseum from that game's bosses. You have to beat the game before you can trade with your GBA, but it's not too difficult.