The third heart is another high one.  This time the heart is reached by making your way across giant floating rocks above this world.  When you enter the gate into Timberdale, stay to your left.  You'll find some stone ledges to hop up that lead to a climbing wall.  Rocks will fall down the left side so be careful.  You'll also pass some enemies on ledges, so watch out for them. 

When you reach the top of the wall, you'll be ready to start hopping across the rocks.  The first one is moving, so wait for it to reach the closest point before leaping.  You'll need to make it across five stones to reach the heart.