I have all 3 regi's, rayquaza, kyogre, groudon, gorebyss and huntail.

to get Huntail&Gorebyss: you need - someone to trade with
- a clamperall [found underwater]
- a deep sea scale/tooth the other person needs the other one [if you get the deep sea scale they need the deep sea tooth]

1. give the deep sea scale/tooth to your clamperall and the person u trade with do the same to there clamperall.
2. trade them over and they will evolve.
3. trade back untill u get the one u want. if u have the one u want to start off with then trade anyway so ure pokedex lists u have both.

Groudon&Kyoger - they're no mystery its easy to solve how to get them.

Rayquaza - is found on top of the sky pillar. in i think route 129. u have to have beaten the elite 4 [E4] and u need a mach bike, master ball/heaps of ultra,great,pokeballs. and a tough team.

regiice,regirock,registeel - they're in acient ruins.

regiice -under the sea [the pokemon emerald guide book give u meanings of braile and the route] you need a wailord and a relicanth.

regirock - found near fortree.

registeel - the desert above mauville city.

HELPFULL- one of the caves.. regirocks or registeels braille reads that u run around the walls of the cave 3 times untill the door opens / were the braille is. one makes u use flash and rocksmash

regiice's braille reads that u dive.

I hope this helps:)