Heart Piece 18
After exiting the first cave, found to the southwest of the skeleton bridge, continue along the path to the fork. There, you can go left or right. Head left and to the end, where you’ll find the Piece of Heart.
Heart Piece 19
Firstly, you’ll need the Mogma Mitts for this. Go to the series of rooms prior to the entrance to the Fire Sanctuary and into the one with the Gossip Stone. Burrow into the hole nearby and go out through the other hole to find a hidden room. The Piece of Heart is on the fairy-occupied ledge.
Heart Piece 20
Plot-related Piece of Heart; you are guaranteed to get this.