To unlock the Hidden Kings trophy in Kingdom Hearts III, you will need to find all 90 of the Lucky Emblems which are spread throughout the game. The Lucky Emblems are the shape of Mickey Mouse's head, and you will have to whip out your Gummiphone and snap a picture of each.
Finding the Lucky Emblems will allow you to view the Kingdom Hearts III secret ending.
You can find video guides for all of the Lucky Emblems below (credit to xTGE on TA/YouTube for putting these together):
Olympus:Twilight Town:Kingdom of Corona:Arendelle:Toy Box:San Fransokyo:Monstropolis:The Caribbean:100 Acre Woods: