HOW TO GET ENTEI, SUICUNE AND RAIKOU:
1) Go to Route 108 and use Dive (requires Surf in the party)

2) Get the Scanner: Head to the submerged control room: =>> http://gyazo.com/5af16f3f6d74ec436c553702d6a85fc5

3) Head to Slateport City and trade the Scanner for the Clear/Tidal Bell, depending on your version.

4) Head BACK to the submerged control room and either Ho-oh or Lugia will appear. Catch it.

5) Put the respective legendary in your party and fly to these locations to catch the legendary beasts

Entei, Suicune and Raikou: Trackless Forest
The Trackless Forest is found by flying on Latios/Latias and is to the southeast of Rustboro City.

The order they appear in rotates EVERY. TWENTY. MINUTES. Go fast and you can actually catch all three within the hour. Good luck

- Synysteria