Oh, use dig at the far end wall. Put Wailord at the beggining of your party, and Relicanth at the end. enter the door made by dig.
Read the braille and then the doors to the REGI's (regiice, regirock and registeel), will be opened. Go to the desert for rock, route from Dewford to Petalburg city to Dewford for ice, and route from fortree to lilycove for steel.