1. Get an orb from bosses or loots
2. Craft an Adamant orb for Dialga by surrounding a normal orb by 8 diamonds/ or Lustrous orb for Palkia by surrounding a normal orb by 8 irons or pearls (which is a new item ) / or Griseous orb for Giratina by surrounding a normal orb by 8 aluminium or another ore
3. Defeat 375 pokemon to fill the orb
4. Find the shrine ( not sure where the shrine need to spawn) then use the orb on it
5. Catch Dialga/Palkia/Giratina
6 Enjoy with them
I hope this idea is good