#1 - Through the Barriers Chest
In the third major section of King's Fall when you're tasked with traversing a barrier by standing on switches and using ships, have two to four of your teammates use the platform to go across the divide. They should then jump and scale the thin side of the wall to the left hand side. At least two members will need to stay behind standing on the switches (they light up when stood on), opening access to the chest area. The other team members will see an opening near the wall on the left side with a door, which they should enter. The chest is immediately ahead. Follow this path back and stand on the switch to allow the other team members to come across and open the chest at the same spot.
#2 - Gorgoroth's Maze Chest
While heading through Gorgoroth's Maze search for a group of four doors with flower covered switches. All four of these need to be stood on in a specific order to access the chest. Bright light will appear on you when you step on the switches in the correct order. There is no punishment for an incorrect order, so try until you get it correct.
One of the doors is difficult to find as it is only accessible through a small opening above a door. A door will open once the correct sequence is completed, revealing the secret chest.
#3 - Transept Jump Puzzle Chest
From the Transept checkpoint, which is at the first platform of the jump puzzle, take out your Ghost to reveal secret platforms. Follow these platforms up carefully until you find a hole in a wall. Cross the gap and you will find the chest in the back of the hidden room.