To a large bowl or the bowl of a stand mixer, add the whipping cream, sugar, and vanilla. Using the whisk attachment, beat cream mixture 3-4 minutes, until stiff peaks form.

Meanwhile, in a medium bowl, combine 1 lb. strawberries, 1 1/2 cups blueberries, and 1 1/2 cups blackberries. Spread a small amount of whipped cream in the bottom of a 9x13-inch glass baking dish. Top with a layer of the graham crackers.

Spoon 1/3 of the remaining whipped cream over the graham crackers and top with 1/3 of berry mixture. Repeat layers of graham crackers, cream and berries twice, finishing with the cream. Arrange the remaining blueberries and blackberries in the top left corner in alternating rows. Arrange the remaining strawberry slices in rows to make the red stripes. Let cake set in the refrigerator for 4 hours or overnight.

Tip: If you don't have fresh berries, frozen work just as well.