At Disc 3 you should be at a point where all the trees are cut down. Find the path that leads to the gate. Open it and start to walk. As soon as you go under the first tree, look to your left. You should see a giant dagger sticking up out of the ground. Walk down and you will find another rolling ball thing. Don't forget what the symbol is and what sound it makes. Return to the path and start walking forwards again. When you reach the two pathways, go right. As you walk down you should see a large fish head. Approach and inspect the head and you will find nothing. However, back on the pathway you should see two lamps. Go until all you can see is the top part. On the very top there is a small disc. Push it and see what happens.