The Brigandine Cape are found in a cave system on the north western side of Quatford. To get in, you’ll need to first blow up the breakable rock barrier using the oil vases nearby. Then, head through the cave system until you reach the chest behind the breakable wooden wall.
The Brigandine Helm is found in Wenlocan Outpost. You’ll need to head into the cave then move the moveable barricade to then enter deeper into the cave, where you’ll then find the wealth chest.
The Sarcophagus Shield is located on the eastern side of Dhustone Quarry. You’ll need to enter through hole in the cave at the bottom of the quarry, then swim under water to the wooden platform on the other side. You’ll find the chest above the water, along with a few pots with other minor loot.