The 3 Regis can only be caught in Ru/Sa/E, and then they need to be migrated from either of those games into D/P. Besides cheating, there is no other way.
If you have Emerald you should do this to get the regies
1
You will need a Pokemon that knows dig, a Relicanth at the end of your party, and a Wailord at the front of your party. You may also want to have a translator for Braille, a language not commonly used but still used today. It is in the form of dots.
2
Get a lot of Ultra balls and Timer balls, then go to the Sealed Chamber (somewhere in the water currents between Slateport and Pacificlog {Route 132-134}). You will need to stay toward the lower part of the route. Eventually, you will see a spot where you can dive. Dive an follow the pathway. When you're there, go to the braille at the north-end of the chamber and use dig. Go through the door and keep walking north to the next braille and press A. An earthquake should be triggered and a door will open at each of the Regi sites.
3
All Regis have Curse, Giga-Impact, AncientPower, and their own special move.
4
To find Regirock, go to the Desert Ruins and enter. It is located toward the bottom of the desert near Mauvile. Go over to the braille and press A to read it. Then go two steps left, then two steps down. Use Rock Smash. Save and go battle Regirock. It has Rock Throw.
5
To find Regice, go to the Ruins on route 105 and enter. Go over to the braille and press A to read it. Staying next to the walls at all times, do a counterclockwise lap (walking, running, or on your bike) around the entire cave. Save and go battle Regice. It has Icy Wind. Be careful, it's icy wind lowers your Pokémon's speed (he will use curse a lot so bring a Pokémon with Yawn).
6
To find Registeel, go to the Ruins on route 120 and enter. You will have to surf under the bridge. Go over to the braille and press A to read it. Go to the middle of the room and use Flash. Save and go battle Registeel. It has Metal Claw.
7
Get 50 ultra balls, and 50 timer balls.Weaken the Pokémon as much as possible and inflict a sleep status to get best results.
8
Remember to save your game before turning off your Gameboy Advanced system or Nintendo DS.hope it helped!
If you have Ruby/Sapphire then you should do this
1
You are going to need a Relicanth at the front of your team, a Wailord at the end of your team, a Pokemon that knows Dive, a Pokemon that knows Surf, and a Pokemon that knows Dig. (None of these are optional.)
2
Go to Pacifidlog Town and use Surf going to the left of town, towards the currents. Stay towards the bottom of the currents.
3
You will eventually see a diving spot. It may take a few tries getting to it. Use Dive there. Follow the path until you see a wall that has text on it. If you read it, it will say in Braille "use dive here".
4
You should now be in a place called the Sealed Chamber. Go to the end of the Sealed Chamber and you will see a wall. It may seem like a dead end, but if you press the A button, it will say in Braille, "use Dig here". Stay facing the wall while you select the Pokemon that can use Dig. If done correctly, the wall will crumble, leaving a passage that you must walk through to proceed.
5
Assuming you have a Relicanth as the first Pokemon in your party and a Wailord as the last, go to the end of the next room, and you will see another wall. It says, "First comes Relecanth. Last comes Wailord." If you have the requirements, press A and an earthquake will happen. A box pops up that says three doors have opened.(you may need to read the braille passage before it will occur)
Locating the Regis
1
Regice: Fly to Petalburg City and go west. Use Surf, and go southwest until you get to an island. Go across it and go north from there and you will discover a cave. Go in the cave, and then read the braille, it says: wait exactly 2 minutes in front of the braille and don't move at all. After waiting, the door will open. Go in and you will find Regice.
2
Registeel: Fly to Fortree City. Go east, then go south, then go west, next go south, then go east, then go south, then go north, then you will find a cave. Go in the cave, then read the braille, it says: go to the middle of the cave then use fly. After doing so, the door will open. Go in, and then you will find Registeel
3
Regirock: Go into the Desert above Mauville City, then look around for a cave. It should be at the bottom of the desert. When you found the cave, go in it, then read the braille, it says: go 2 steps right, then 2 steps down, then use strength. After doing so, the door will open. Go in, then you will find Regirock.
4
Names of Caves: Regice - Island Cave
Regirock - Desert Ruins
Registeel - Ancient Tomb
After this you just have to migrate them to Pokemon Diamond/Pearl/Platinum(u can catch them in platinum after you have the special regigigas)/SoulSilver/Heatgold
And this is how to migrate Pokemon:
First you have to beat the game which is by beating the elite 4. once that happens you have to see 150 Pokemon in the game. once you see all 150 you go to the professor and he will give you a national pokedex. then pal park with be available. restart the game insert the gba game you want and then go to the migrate from the game (in the main menu). choose 6 Pokemon that are in ur boxes. then go to pal park and catch the ones you took out of the gba there.
kind of a long process but its worth it. good luck
Hope I helped :D
