In a large mixing bowl, whip the heavy cream until soft peaks form. Gradually add the powdered sugar and vanilla extract, continuing to whip until stiff peaks form.

In a 9x13 inch baking dish, spread a thin layer of the whipped cream mixture on the bottom.

Place a layer of graham crackers over the whipped cream, breaking them as needed to fit.

Spread half of the remaining whipped cream mixture over the graham crackers.

Layer half of the sliced strawberries, blueberries, and raspberries on top of the whipped cream.

Repeat the layers: add another layer of graham crackers, the remaining whipped cream mixture, and the remaining berries.

Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.

Before serving, garnish with additional berries if desired.