Ok this is really long so listen carefully.
After you've beaten the Elite Four go to the Weather Institute and go upstairs and talk to the scientist guy and he'll either say that there has been a drought that means you'll get Groudon or he'll say that there has been hevy rainfall.
Groudon:
If the scientist said that there has been a drought he'll say a Route.
If he says Route 116 go to Route 116 its near Rusturf Tunnel at Rustboro's side or at Vendanturf's side the cave that Groudon's in is random so go check every little mountain to see if there are any caves if its not at Rustboro go to Vendanturf and then go inside the cave and then go down and then you'll come out through another side and you'll see a guy there and if you see a cave go in and then you'll see Groudon.
Route 114 is near Fallarbor Town do the same thing look at every Rock and mountain and eventually you'll find the cave.
Route 115 To get to Route 115 go to Rustboro and keep on going up until you'll get to a patch of water well use Surf and you'll get to another place and you'll see a couple of mountains well search until you find a cave.
Route 118 is near the place where you have to Surf from Mauville to Fortree so now use Surf or the cave might be next to Mauville or the other side so now use Surf and get to the other side the cave might be next to the Sitrus trees.
Kyogre:
To get to Kyogre the scientist has to say that there has been a heavy rainfall and he'll say where it is so follow the Route the scientist says and go to the Rout use Surf and there will be a black thing which means you can use Dive there so DIve and you'll find Kyogre.
Oh yeah and you can get both in this game.
thank you.i have caught all three of them groudon kyogre and Rayquaza.