In a large mixing bowl, whip the heavy cream with an electric mixer on medium speed until it starts to thicken. Gradually add the powdered sugar and vanilla extract, then continue to whip until stiff peaks form.

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

Layer 6 graham crackers over the whipped cream, breaking them if necessary to fit.

Spread half of the remaining whipped cream over the graham crackers, then layer half of the sliced strawberries, blueberries, and raspberries on top.

Repeat the layers by adding another 6 graham crackers, the remaining whipped cream, and the rest of the berries.

Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results, to allow the graham crackers to soften.

Before serving, garnish with fresh mint leaves if desired.