The three Regis are Regirock, Regice and Registeel. These Legendary Golems can make the final phase of the game a lot easier, as you can catch them before your first attempt at defeating the Elite Four. However, your mission to defeat all three Pokemon may take a while and will require you to travel to the Hoenn region.
Steps
Part 1 of 7: Preparing to unlock

Step 1. Bring Pokemon that can dig (Dig), surf (Surf) and dive (Dive)
These three skills are essential to unlock the three Regis. You can get Dig from the brother of the Fossil Maniac on Route 114. HM03 (Surf) is obtained by beating Norman, the Petalburg Gym leader. Dive can be found with Steven in Mossdeep City.

Step 2. Beat the seventh Gym
You will need to defeat at least the first seven gyms to gain all the skills you need to defeat the Regis. After defeating Tate & Liza in the Mossdeep Gym, you will also be able to use Dive in deep water. This is the last skill you need to unlock the Regis.

Step 3. Catch a Relicanth
This is a rare Pokemon that can only be found underwater. In Ruby and Sapphire, you can use Dive on Routes 124 and 126 to encounter a Relicanth. The chance that the Pokemon will appear is 5%. In Alpha Sapphire and Omega Ruby, you can also find underwater areas on Routes 107, 129, and 130. There are several underwater areas scattered around the world.

Step 4. Catch a Wailord or have your Wailmer evolve into a Wailord
A Wailmer can be caught in various places in the game. When Wailmer reaches level 40, he evolves into Wailord. You can also try to catch a Wailord right away by surfing around Route 129. However, the chance of encountering the Pokemon here is only 1%.
Part 2 of 7: Surfing the waves

Step #1. Go to Pacifidlog Town
This is the easiest place to start your Surf. If there are no more good waves here, you can always come back later to try again.

Step 2. Surf west from Pacifidlog
You will soon end up on Route 132.

Step 3. Head south onto the island
Walk around the island and then surf to the shallow spot. Walk to the other side and then continue surfing from the southwest of the shallow spot.

Step 4. Keep surfing west until you reach the next shallow spot
Walk to the westernmost spot of the island. Make sure you stay at least three steps from the coast and also at least three steps from the lower coastal edge.

Step 5. Surf west off the island
The waves should take you straight to the dark spot in the water. Now use Dive to explore the underwater world.
Part 3 of 7: Unlocking the Regis

Step 1. Go to the dark spot in the water and use Dive
You can find the dark spot at the bottom right of Route 134. There are six rocks around the place.

Step 2. Surf south through the cave until you find the Braille tablet
You will find this on the other side of the bend in the cave.

Step 3. For the tablet, use Dive
You will now enter the Sealed Chamber.

Step 4. The Braille tablet you need is against the back wall of the cave
All kinds of inscriptions can be found in the cave. However, the one you need is the one against the back wall of the cave. The inscription reads "DIG HERE".

Step 5. Use Dig for the Braille tablet
Walk up to the inscription and look at it. Now use Dig. This will make a door appear in the wall.

Step 6. Make sure Relicanth is the first Pokemon in the group and Wailord the last
You will need to put Relicanth at the top of your Pokemon list and Wailord at the bottom. Change the order of your Pokemon in the appropriate screen.

Step 7. Look at the Braille tablet in the back of the second cave
If Relicanth is at the top of your Pokemon list and Wailord at the bottom, an earthquake is now happening. This means that the doors to the Island Cave, Desert Ruins and the Ancient Tomb have been opened all over the world. These three doors will lead you to the three Regis.
Part 4 of 7: Catching Regice

Step 1. Prepare your Pokemon
Regice is a level 40 Pokemon, but it is wise to make sure that the Pokemon you want to bet have reached at least level 50. A common strategy for waking Regice without knocking him unconscious is to have a Pokemon use the False Swipes attack. This makes the enemy weaker, but not so weak that he is knocked unconscious. Another common strategy is to use the Sleep or Paralyze attacks. This increases the chance of catching the Pokemon.

Step 2. Make sure you have more than enough Ultra Balls with you
Ultra Balls are best suited for catching Regice. How many balls you need to catch him depends on Regice's health and status.

Step 3. Go to Route 105
The Island Cave on the western side of the route will now be open. Enter this cave.

Step 4. Read the inscription and wait
According to the instructions you will have to wait "twice". This means that you have to wait two minutes without moving. You can put your controller down for a while to make sure you don't accidentally move. If you do, you will have to leave the cave and try again. After two minutes a door will open.

Step 5. Save the game
You only have one chance to fight with Regice, so it's a good idea to save your game before the fight. For example, you can start over if Regice becomes unconscious or you are defeated by the Pokemon.

Step 6. Capture Regice
Walk up to Regice and start the fight. Reduce the Pokemon's powers, then use the Sleep or Paralyze attack. Once Regice is weak enough and either asleep or paralyzed, you can start throwing Ultra Balls at him.

Step 7. Rename Regice (Alpha Sapphire and Omega Ruby)
If you plan to catch Regigigas as well, you'll need to rename Regice. You can do this once you've caught him, but you can also take Regice to a Name Rater to get a new name. The Name Rater is located south of the Contest Hall in Slateport City.
Part 5 of 7: Catching Regirock

Step 1. Have a Pokemon with you that has the Strength ability
This is required to gain access to Regirock.

Step 2. Prepare to fight with Regirock
Like the other Regis, Regirock is a level 40 Pokemon. Bring water Pokemon to weaken it as effectively as possible. Also bring a Pokemon that knows the Sleep or Paralyze attacks and a Pokemon that knows the False Swipes move. Also make sure you have more than enough Ultra Balls with you.

Step 3. Go to Route 111
The Desert Ruins are on the eastern side of the route.

Step 4. Read the inscriptions on the back wall of the cave
Here's exactly what you need to do to get into Regirock.
- Take two steps to the right and then two steps down.
- Now use Strength to make the door appear.

Step 5. Save the game
You only have one chance to beat Regirock, so be sure to save your game before starting the fight. That way you can start over if something goes wrong.

Step 6. Catch Regirock
Take on the fight with Regirock. Use your water Pokemon to quickly weaken Regirock, then use the False Swipes attack to make him even weaker without knocking him out. Use the Sleep or Paralyze attack, then start throwing Ultra Balls.
Part 6 of 7: Catching Registeel

Step 1. Bring a Pokemon that has the Fly ability
You need this to open the door to Registeel.

Step 2. Prepare for the fight with Registeel
Like the other two Regis, Registeel is a Legendary Pokemon at level 40. It's a Steel Pokemon, making it hard to weaken. Also, make sure you have Fighting, Earth, and Fire Pokemon with you to weaken Registeel as effectively as possible. Also bring a Pokemon that knows the False Swipes move. This way you can weaken Registeel, but at the same time prevent him from becoming unconscious. Optionally, you can also bring a Pokemon that knows the attacks Sleep or Paralyze so that you can catch Registeel more easily. Also don't forget your Ultra Balls!

Step 3. Go to Route 120
The cave of the Ancient Ruins is on the west side of the route.

Step 4. Read the inscriptions and then stand in the middle of the room
On the wall you can read exactly how to access Registeel.
- Stand in the middle of the room.
- Use Fly to open the door.

Step 5. Save the game before starting the battle
You only have one chance to catch Registeel, so you'd better save the game first. Then you can easily start over if something goes wrong.

Step 6. Capture Registeel
Go into battle and try to weaken Registeel as quickly as possible by deploying your Fighting, Earth and Fire Pokemon. Use False Swipes to weaken Registeel without knocking him out. Once Registeel is weakened enough, you can use Sleep or Paralyze and then catch him with Ultra Balls.
Part 7 of 7: Catching Regigigas (Alpha Sapphire and Omega Ruby)

Step 1. Defeat Groudon or Kyogre
You will not encounter Regigigas until you defeat this Legendary Pokemon.

Step 2. Make sure you have captured the three Legendary Golems
You will need Regirock, Registeel and Regice to encounter Regigigas.

Step 3. Give Regice a nickname
If you didn't give Regice a nickname when you caught him, you'll need to travel to Slateport City to talk to the Name Rater. You can give Regice any name you want, as long as it's a new name.

Step 4. Give your Regice a "cold" object to hold
Your Regice needs a Snowball, an Icy Rock, a Never-Melt Ice, or a Casteliacone.
- Snowballs can be won at the Battle Maison on Snorunts.
- Icy Rocks can be found at the Weather Institute.
- Never-Melt Ice can be found in Shoal Cave.
- Casteliacones can be won in the Contest Hall or bought in Mauville City. To buy them, you must first have captured Regice.

Step 5. Go to the Island Cave on Route 105
In this cave you have also found Regice. Make sure to visit the cave during the day, as you won't encounter Regigigas here at night.

Step 6. Save the game before entering the cave
The battle starts as soon as you enter the cave, if you meet the necessary requirements. You only get one chance to beat Regigigas, so it's a good idea to save the game first.

Step 7. Catch Regigigas
Unlike the three Golems, Regigigas has level 50. A tough fight awaits you. Regigigas are best defeated with fighting Pokemon, as these weaken him the fastest. Once Regigigas is weakened, you can use False Swipes to make him just a little weaker, but not knock him out. Then use Ultra Balls to capture Regigigas.