Instructions
Regirock
Catch a Ground or Rock type, such as Diglett and its evolutions, and teach it "Strength" and "Dig." Obtain a Wailord and a Relicanth by swimming in the seaweed underwater. Go to Route 134, which has many currents.
Travel from the east side of Route 134 to the southwest side. Eventually, you will reach land that you can dive into. Use "Dive" and follow the path until you come upon an inscription in Braille. Resurface and you will find yourself in a cave. Read the inscription in the cave and use "Dig."
Make Relicanth your lead Pokemon and put Wailord in the last slot of your party. Reach the inscription and the cave will open. Head to Route 111 and enter the now open cave. Inspect the inscription. Walk right twice, down twice and use "Strength," which will cause Regirock to appear.
Regice
Fly to Dewford Town or Petalburg City. Use "Surf" to travel to Route 105. You will surf southwest if you flew to Petalburg City or northwest if you flew to Dewford Town. Travel within Route 105 until you encounter a cave similar to the one where you found Regirock.
Enter the cave and walk in the same direction until you reach the northern wall. Inspect the inscription by pressing "A" and wait for a few minutes. Eventually, a door will appear.
Save your game and go inside the room. You will find Regice, which is an Ice type, at level 40.
Registeel
Fly to Lilycove City after capturing Regirock and Regice. Buy extra Ultra balls if you have less than 40 to 50. Head west of Lilycove until you are in front of the Safari Zone on Route 121.
Continue walking west instead of entering the Safari Zone. Eventually, you will encounter a staircase that wasn't there before. Walk up the staircase and you will arrive in another rainy part of Route 120.
Look for a mountain that is similar to the previous ones where you found the two other Regis. Enter the mountain and read the inscription. Walk to the very center of the mountain and use "Fly." Go inside the door that appears to find Registeel.