This page explains how to find the Dow Na'eh shrine and solve the "Three Boxes" puzzle.
How to Find the Shrine
Go to Ta'loh Naeg Shrine and go east into the forest. Go northeast to Lanayru Road - West Gate, and go southeast at the fork. You will reach an area of the water. Go east through this area, and look for a waterfall to the south. The shrine is behind the waterfall, so swim past the waterfall, then climb up the rocks.
How to Solve the Shrine
There is a treasure chest on a wooden platform to the left of the large switch across from the elevator where you entered the shrine. Shoot the two ropes holding up the platform. Then use Cryonis to get close enough to the chest to use Magnesis on it. It contains a Zora Sword.
There is another magnetic treasure chest in the water, between the large switch and the platform that rises and falls. Use Magnesis to take it out of the water. It contains an Opal.
Then open the chest on the platform that rises and falls. It contains amber.
Now use Magnesis to put all three magnetic chests on the large switch. The weight of all three chests will push the switch down. This opens some bars under Dow Na'eh, allowing you to pull out the large magnetic box. Use Magnesis to put the box onto the platform that rises and falls. Then stand on the rising-and-falling platform and get off at the platform on top. Then when the platform goes back down, float onto the magnetic box. When you go back up, climb onto the raised walkway, and walk to Dow Na'eh from there.