Prepare the Semi-Sweet Chocolate Layer: In a mixing bowl, combine melted semi-sweet chocolate with 1 box of cream and 1 tablespoon of dissolved gelatin (following package instructions). Mix well until creamy.
First Layer: Spread this mixture into the bottom of a medium loaf pan lined with plastic wrap. Chill in the refrigerator for 1 hour until set.
Milk Chocolate Layer: In another bowl, mix melted milk chocolate with another box of cream and 1 tablespoon of dissolved gelatin. Spread this layer atop the semi-sweet chocolate layer and refrigerate for another hour.
White Chocolate Layer: Finally, combine melted white chocolate, the last box of cream, and the remaining gelatin. Spread this mixture over the milk chocolate layer and refrigerate for 2 hours.
Serve: Once set, turn out the mousse onto a platter, decorate with chocolate shavings, and serve.