Allow me to help.
Basically, you get a Wailord, and a Relicanth.
Here's how you obtain these...
Wailord: Fish up a Wailmer from anywhere really, and evolve it at level 40.
Relicanth: Outside Sootopolis, in the underwater grass. It's rare, so I generally lure it with Sweet Scent.
Get those Pokemon, plus one that knows Dig, Surf and Dive, and you're all set. Go to Pacifidilog Town, the floating one around route 130, and surf the currents until route 134. You'll then, if you've done it right, get to a decent patch of water. Stand in middle of it, and surf, and you should get to a deep patch of water where you can dive. Do so. Then surf around until you get to a spot of Braille, and surface from the water. All the Braille says is "Go up here".
OK, enter the cave, and walk directly forward until you hit the wall (ignore the braille rocks... I spent hours trying to translate the buggers, only to discover they spelt out the sodding alphabet), then dig the wall, which will cave in. Carry on walking in a straight line, and read the braille on the top wall (don't bother deciphering it... takes years, and is pointless), but by reading it, you trigger an earthquake. You will then hear three doors opening.
Congratulations! You've unlocked the three elusive little gits lol. Now to get them.
Registeel = My personal favourite lol. Fly to Lilycove, buy shedloads of Ultra and/or Timer Balls, get out of the city, follow the road around and you'll come to a girl who gives you Iapapa or Fijy berries every day. There's a staircase near her; go up it, through the tall grass, and you'll find a fossil hunting trainer, who is, fortunately for you, blissfully unaware that the giant rock in the area in which he searches in open for your perusal.
Go in the door, and save the game. Do it now... it's easy, and it might save you hassle lol.
Now, I can't quite remember the precise directions to go in (I haven't done this for a while), BUT I THINK IT'S THE FOLLOWING...
1. Go two steps up.
2. Then three right.
3. Then use fly.
If that works, a doorway will open up ahead. Go through it, and there's Registeel.
FOR THE LOVE OF DEOXYS, SAVE THE GAME BEFORE YOU TRY AND CATCH IT! If you accidently kill it, you won't get another shot at it, so save the thing first.
Alright, moving on...
Regirock: This is slightly easier to get, I think.
Go to the desert, and to the big rock in the near the bottom of it. The doorway will be open, where it was previously sealed.
Now, it seems obvious... but go in lol.
I really cannot recall this very well, so you may well want to double check this...
1. Go four steps straight ahead.
2. Then two right.
3. Use Strength.
Once again, if successful, the doorway will open. Save the game again, obviously.
And last but certainly not least, we have Regice.
Regice: OK, this one doesn't involve directions, just patience and a little more navigation to get there in the first place lol.
Where you get the boat to Dewford, you pass a lot of trainers and rocks. There's a vast island of trees, just after where you board the ship from Briney's. Anyway, go there, and navigate your way through the rocks, until you'll eventually wind up at a rock that was previously closed. DO NOT GO IN JUST YET. Unless you're VERY patient. Save the game here. Then walk in, straight ahead, up to the wall, and press the A Button. That's it. And then, you just wait for the door to open... don't move from that spot, and don't save the game until the door opens, or it won't work. It generally takes about 15 minutes. I suggest getting some coffee at this point lol.
EVENTUALLY, the door will open, and you'll be face to face with Regice. Save the game, catch the git, and claim yourself the best Regi trainer ever! =D
OK, here are some points to remember.
1. Use types that are strong against the opposition, AKA the Regis... Ice for Registeel, or Dragon, Water or Grass for Regirock, and Fire for Regice. However, don't go overboard... you don't want them fainting on you. Status attacks really help.
2. ON NO ACCOUNT DO YOU USE THE MASTER BALL AGAINST ANY OF THEM! It's not worth it. Save that for Rayquaza, Latias, or, if you plan on getting the Eon Ticket, Mew. I made that mistake, and it annoyed me a heck of a lot lol. Use Timer or Ultra Balls, and have patience. It may take several tries to get the tenacious trio.
3. These guys aren't legendaries for nothing. They hit hard, and they hit fast. Stay on your guard unless you want your Primary attacking Pokemon fainting.
4. This is just a general tip... don't give them stupid names. They deserve their original ones lol.
All that's left for me to say is... good luck with this, and I hope you "Catch 'em all!"
Hopefully, this will help at least partially.
Shiny xxx