[This is the list of all legendaries][1]
>"When Soaring in the Sky, if you have Lugia or Ho-Oh in your team, then a new area will open up. This area can only be accessed when one of the Pokémon is in your team. This area is located east of Rustboro City and is a small forest. In there, you will find a portal which has Raikou, Entei or Suicune in. For Raikou, the time needs to be between 00 and 20 minutes past the hour, for Entei the time needs to be between 20 and 40 minutes past the hour and for Suicune, the time needs to be between 40 and 00 minutes past the hour."
Hope this helps
[1]: http://www.serebii.net/omegarubyalphasapphire/legendary.shtml
