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You have to go to that cave first on that route that has the speedy water. I don't remember the number of the route, but I do know ya need Dive. You also need a Wailord and a Relicanth. Put either the Wailord on top and the Relicanth at the bottom, or vice-versa depending on the version you're playing. Then use like Rock Smash or Strenght or Flash or something and go in. The regis activate.
Next, to catch each regi, you have to go to each cave and do specific things.
Regirock's cave is in the desert. Go there and use either Strenght or Rock Smash too, or something. Then you can go in and there's a Regirock with that awesome music. And you catch, there ya go, one down, two to go.
Regice's cave is in the water route between Mr. Peecko's house and the city with the second, fighting type Gym. Go in there, and just wait. That's it. Wait 30 seconds. Then you can go in and catch the blasted thing. Two down, one to go.
Registeel's cave is in the route after the flying type gym's city. Bring some one with Flash and a Mach Bike too. Go there and use Flash, the Regi appears, and voilá.
I totally remebered it, dude.