1. Mogi's third letter
Make your way to the peninsula over the wooden bridge, that you can find in the area marked in the screenshot with the red circle.
Approach the area marked as one, where you will find a bunker. Enter through the entrance on the western wall, shown in the screenshot, and go down the stairs.
Downstairs, take the first passage to the left, glowing green. In the corner, there is the corpse with the letter.
2. Hayato's first letter
Go down to the beach in the place marked with the circle, to face a cave entrance.
The corpse, and the letter, is at the end of the tunnel.
3. Swinji's letter
Reach the area marked with the circle in the screenshot. You will find a descent to your right, which you need to take.
The entrance to the bunker is right after you turn the corner to the left.