Heat the oven to 350°F and grease a 9-by-13-inch baking pan. Prepare the lemon cake mix with the 3 eggs, 1/2 cup vegetable oil, and 1 cup water, following the package directions, then pour the batter into the prepared pan.

Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10–15 minutes so it remains warm but is not piping hot.

Use the handle of a wooden spoon or a large straw to poke holes across the cake, spacing them about 1 inch apart. Whisk the instant lemon pudding mix with 2 cups cold milk for about 2 minutes, until thickened.

Pour or spread the lemon pudding over the warm cake, guiding it into the holes with a spatula. Smooth the top, cover the pan, and refrigerate for 1–2 hours, until the pudding is set and the cake is thoroughly chilled.

Spread the thawed whipped topping over the chilled cake. Scatter 2 cups fresh blueberries across the top, then add the optional lemon zest and a small drizzle of lemon juice just before serving.