In a large mixing bowl, using a handheld electric mixer, beat together cream cheese and instant pudding until combined. Slowly mix in the milk. Beat on medium speed for 2 minutes. Fold in the Cool Whip. Spread a spoonful of pudding mixture on the bottom of a 9 X 13-inch dish in a thin layer.
Over the thin layer of pudding mix, line the pan with a single layer of graham crackers, cutting or breaking them as need be, to cover completely. Spread 1/3 the pudding mixture over the layer of graham crackers, then add 1/3 strawberries, raspberries, and blueberries
Place another layer of graham crackers, cutting them to fit. Spread 1/3 pudding mixture over top and add another 1/3 of the berries in a layer.
Top with the final layer of graham crackers. Spread the reserved pudding mixture over top and even out.
Cover and refrigerate a minimum of 4 hours to (better) overnight, or until the graham crackers have completely softened.
Before serving, top decoratively with the remaining 1/3 of the berries. Cut into pieces and serve with a small sprig of mint.