If all three have been obtained, each of the crowns will be given a new effect after a short cutscene. Talk to King Vendrick, and for each DLC crown obtained, there will be a specific dialog.
Interact with King Vendrick's armor in the chamber at the end of the Undead Crypt to enter the Memory Of The King. Reach the Ancient Dragon, and obtain the Ashen Mist Heart. Uncover the hollowed King Vendrick in the Undead Crypt, get the King's Ring, and enter Aldia's Keep. Open the gates at the Shrine Of Winter, and enter Drangleic Castle. Then, enter the Memory Of The King, using the following steps. Obtain the 'Crown Of The Sunken King', 'Crown Of The Old Iron King', and 'Crown Of The Ivory King' from their respective DLCs. Vendrick's Blessing and True King's Crown