Regice: cave on Route 105. Go to the Braille section, it says "Stay close to the wall. Run around one lap." So you walk round in a circle, alongside the wall.
Regirock: cave in the Desert Ruins. The Braille says "Left, Left, Down, Down. Then use Rock Smash." So go 2 steps left and 2 down and use Rock Smash.
Registeel: cave on Route 120. The Braille says "Those who inherit our will, shine in the middle." So go to the middle of the room and use Flash.
And Regigigas is not in Emerald, it was introduced in Gen 4.