Prepare the pound cake according to the recipe directions. Let cool completely, then cut half of the cake into 1 1/2-inch cubes. Save the remaining cake for another use.
Combine in a medium bowl and whisk the pudding mix into the milk until it begins to thicken, about 2 minutes. Fold in the whipped topping. Cover and refrigerate for 1 hour.
Meanwhile, in a large bowl, toss together the strawberries, blueberries, blackberries, and sugar substitute. Layer half of the cake cubes in the bottom of a clear trifle bowl. Top with a thrid of the berries, then half of the pudding. Repeat the layers of cake, berries, and pudding, topping with a final layer of berries.