Scoop out about 1/2 cup of juice from the strawberries. Stir in the cornstarch. Pour into a pan with the rest of the strawberries. Heat on the stove on medium high until thickened, stirring constantly. Stir in 1 tablespoon of the cream sherry. Set aside to cool.
Mix 1/2 cup sugar, flour and salt in top of double boiler. Blend in 3 egg yolks and milk. Cook, uncovered, over boiling water, stirring constantly for 10 to 12 minutes or until thickened. Remove
from heat. Cover with plastic wrap touching custard. Let it cool in refrigerator for about 30 minutes. Pour cooled custard in bottom of 13x9 glass casserole dish. Place pound cake slices on top of custard. Brush remaining 1/3 cup of cream sherry onto cake slices. Scoop spoonfuls of cooled strawberry mixture on top of cake, distributing evenly. Add 1 cup of blueberries and 1 cup of raspberries. Spread whipped topping over berries. Create a flag decoration with the remaining berries. Let it chill in the refrigerator for 1 hour before serving.