How do you open the three regis caves?

First, get a Pokemon with Surf and Dive, a Pokemon that knows Dig, and a Wailord and Relicanth.
Go to Pacifidlog Town and go west into Routes 132-134. There are strong currents on this route and the main idea is to stay as low as possible on the screen. It's a little tricky to get to where you want to go, but it is possible a couple of ways. Where you want to go is in a deep area on Route 134 where you can Dive in. Once you find a way there, go underwater and follow the path to a rock that reads "Go up here". If you're standing directly in front of these "markings" (Braille)... Go up here! At the back of the Sealed Chamber is a wall with more braille that reads "Dig Here". So, use the move Dig there. The door will open that leads to another room. In the back of THAT room, there's ANOTHER wall with braille that reads "First comes Wailord, Last comes Relicanth". So put Wailord first in your party and Relicanth last in your party, and when you get out of that, the screen will shake and you've unlocked the Regi's caves on Routes 105 (Regice), 111 (Regirock) and 120 (Registeel).

In the Regice Cave, run one lap around the room touching the walls, but if you bump into the wall (when you hear a little *bump*), you have to start over again. Walk slowly if you have to. The door will open, and so you enter and you'll see the Regice.

In the Regirock Cave, simply go left two times and down two times (starting in front of the markings) and then use Rock Smash. Once again, the cave will open and you'll see the Regirock.

In the Registeel Cave, just go to the center of the room and use Flash. What else is new, the cave will open that leads to a Registeel.