obtaining Arceus: you must first obtain the Azure flute and complete the game. once that is done, go straigh to MT. cornett and make your way up to the spear pillar and when you walk on the tile in front of the entrance the game will say that the azure flute is making a weird sound. then a pair of magical stairs will come down and arceus is at the top waiting. Arceus is Lv 80 and a normal type.
obtaining Darkrei: to obtain Pokemon darkrei you must obtain the Membersip card. once you have obtained it, go to canalve city and there should be an abandoned house straight up from the pokemon center. If you already tried going in there and im sure you couldn't have gone in. but now the door will be open. there will then be a man with glasses standing in front of you saying that they had a reservation for you waiting. he will then walk you to the bed and lie you down and you will then fall asleep. next u will wake up on an island. go to the entrance of the trees and there Darkrei is waiting. Darkrei is at Lv 40 and is a Ghost type.
obtaining Girantina: to obtain Pokemon Girantina. you must first complete the game. one its completed. go to the hotel where a lady lost her key. near there keep going up and follow the path against the right side of the forest trees passing the lake where team Galactic put a bomb in. once going across on the right there should be a path that was never open before. go in the path and the path is called the ''Spring Path''. keep going and follow the trail. once far enough you will then see a bridge and under a small lake of water. reach down there and go across and there should be a cave entrance. once in the cave its a mase with a series of doors all around you. go though any and keep going through the open cave holes and soon enough you will come across a big pillar. in front standing of the pillar is a pokemon called girantina. Girantina is Lv 70 and a Ghost type.