Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, combine the cake mix, eggs, 1/3 cup oil, 3 zested lemons, 1/4 cup fresh lemon juice, 1/3 cup milk, and 2/3 cup sour cream; stir until smooth and well combined.
Pour the batter into the prepared pan and bake 25–30 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
While the cake bakes, dissolve the 3-ounce lemon Jell-O in 1 cup boiling water, stir to combine, and let the mixture cool to room temperature.
When the cake is still warm, use the back of a wooden spoon or a skewer to poke holes evenly all over the top of the cake.
Pour the cooled lemon Jell-O mixture over the warm cake, allowing it to soak into the holes; spread gently so it penetrates evenly.
Spread 2 cups lemon curd evenly over the cake surface.
In a bowl, whisk the lemon instant pudding mix with 2 cups cold milk until thickened, then immediately pour the pudding over the cake and spread to cover.
Refrigerate the cake for several hours or preferably overnight to set and allow flavors to meld.
Before serving, beat 1 pint heavy whipping cream with 1 tablespoon lemon zest and 1/2 cup powdered sugar until stiff peaks form, then spread the whipped cream over the chilled cake.
Chill the cake an additional hour if desired, slice into 12 pieces, and serve.