How the heck do you get regirock, registeel, and regice and where?

Regirock, Regice, and Registeel:

Let's say for a second that the three titans you're looking for aren't hard to find, but they are extremely hard to capture.

You must possess a Relicanth and Wailord in order to do this. You can obtain a Relicanth around Sootopolis City and a Wailord by fishing on Route 129 or evolving a Wailmer at level 40.

At any time after you defeat the Mossdeep City Gym, travel to Pacifidlog Town, and then manipulate the current to end up on a small dive spot. Use Dive and travel underwater; you will come across a small mound with a Braille inscription on it: GO UP HERE. Use Dive directly in front of the inscription; you'll be in the Sealed Chamber.

Walk to the Braille inscription "DIG HERE" at the back of the first chamber and have a Pokémon use Dig right in front of it. You will access a second chamber. Walk to the iscription and arrange your party so that Relicanth is to the left and Wailord is in the last position in your party. When you press A in front of the inscription, a tremor will occur and the display will read, "It sounds like a door opened very far away."

Exit the Sealed Chamber and fly to Petalburg City, then Surf on Route 105. To your left you will see a little monolith with a hole in it. Find the route to the monolith and enter it. In the Island Cave, walk up to the middle of the inscription ("STOP AND WAIT. WAIT FOR TIME TO PASS TWICE"), and then don't do anything for the next two minutes. The wall will collapse, and you can attack Regice now.

Regice (Level 40): Icy Wind, Superpower, Ancientpower, Curse

To obtain Regirock, go down to the Desert and look for another monolith the same size as the Island Cave. Enter, walk to the inscription and press A, walk two paces to the right and two more down, and then use Strength. (If none of your Pokémon know Strength, you may teach it now.) The wall will crumble, leaving you free to attack Regirock.

Regirock (level 40): Rock Throw, Superpower, Ancientpower, Curse

When you're looking for Absol on Route 120, look for a flight of steps by the pond (not the one with the Scorched Slab) and ascend it. You'll find the Ancient Tomb. Enter the monolith, proceed to the EXACT center of the chamber, and use Fly. The wall will collapse, so you can capture Registeel.

Registeel (level 40): Metal Claw, Superpower, Ancientpower, Curse

Note #1: Only Regirock, Regice, and Registeel can learn Superpower.
Note #2: As these are extremely hard to capture (you may need to carry 20 Ultra Balls for each one!), I recommend using the Master Ball on any one of them, if you have it.

Illumise:

You say you have trouble looking for Illumise on Route 117. Just keep looking in all of the grassy areas. If push comes to shove, look for one in the Ruby version; they're easier to find there.

As for Feebas and Chimecho, I have unreliable ideas about Feebas, I forget how to obtain the two items you asked for, and don't know how to obtain Chimecho. Sorry!