FOR THE FILLING: Whisk 1/4 cup sugar and 1 tablespoon pectin together in small saucepan. Add 1 1/2 cups blueberries and mash well with potato masher. Bring mixture to boil over medium heat and cook, stirring frequently, until starting to thicken, about 2 minutes. Strain mixture through fine-mesh strainer set over bowl, pressing on solids to extract juice; discard solids. Cover with plastic wrap and refrigerate until set, about 2 hours. Rinse saucepan and repeat mashing and cooking process with remaining 1/4 cup sugar, remaining 4 teaspoons pectin, 1 cup strawberries, and 1 cup raspberries. Strain and press over separate bowl (discard solids), cover with plastic, and refrigerate until set, about 2 hours.

TO ASSEMBLE: Using long serrated knife, cut 2 horizontal lines around sides of cake; then, following scored lines, cut cake into 3 even layers. Place 1 cake layer on platter. Spread blueberry filling evenly over top, followed by 1 1/2 cups whipped cream. Top with second cake layer, press lightly to adhere, then spread strawberry-raspberry filling evenly over top, followed by 1 1/2 cups whipped cream. Top with remaining cake layer and spread remaining 1 cup whipped cream on top (do not frost sides). Toss remaining 1/4 cup blueberries, 1/4 cup strawberries, and 1/4 cup raspberries together and arrange in small pile in center of cake. Serve.

Makes One Cake