Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the lemon cake mix, 3 large eggs, 1/2 cup vegetable oil, and 1 cup water. Mix until smooth and well combined.
Pour the batter into a greased 9×13 inch baking pan and bake for 28-30 minutes or until a toothpick inserted into the center comes out clean.
Allow the cake to cool completely in the pan on a wire rack.
Once cooled, cut the lemon cake into 1-inch cubes.
In a large bowl, whip 2 cups heavy whipping cream with 1/4 cup powdered sugar and 1 teaspoon vanilla extract until stiff peaks form.
In a large trifle bowl or individual serving glasses, start layering by placing a layer of lemon cake cubes at the bottom.
Spread a layer of lemon curd (about 2 tablespoons per layer) evenly over the cake cubes.
Add a layer of whipped cream over the lemon curd.
Sprinkle some fresh blueberries and raspberries over the whipped cream.
Repeat the layering process (cake cubes, lemon curd, whipped cream, berries) until the bowl or glasses are filled, finishing with a whipped cream layer on top.
Garnish the top with remaining fresh berries and sprinkle 1 tablespoon lemon zest over the whipped cream.
Refrigerate the trifle for at least 2 hours before serving to allow flavors to meld.