Prepare the crust. Place the oreo cookies into the bowl of a food processor and crush until crumbs form. Add melted butter and process until evenly moistened. Press the mixture into the bottom of a greased 8 inch (20 cm) springform pan using the back of a spoon. Refrigerate until the first layer of mousse is prepared. Prepare dark chocolate mousse. In a heat proof bowl add the semi-sweet chocolate and 1/2 cup (120g) whipping cream. Place the bowl over a saucepan with simmering water, over low heat, until all the chocolate is melted. Meanwhile dissolve gelatin in cold water and let it swell for about 5 to 10 minutes. Place the gelatin over low heat just until the gelatin dissolves and pour it over the melted chocolate mixture. Let the chocolate mixture cool completely at room temperature. Whip the remaining 2/3 cup (160g) chilled whipping cream until stiff peaks form. Add melted chocolate mixture and mix until well combined. Pour the chocolate mousse over the crust and refrigerate for about 15-20 minutes to set slightly, until you prepare the next layer of chocolate mousse. Repeat the same steps for preparing the milk and white chocolate mousse layers. Refrigerate the cake for at least 4 hours or overnight to set. Run a knife around the edges of the cake to remove the sides of the pan. Before serving dust the top of the cake with cocoa powder and decorate with fresh berries if desired. Enjoy!