1. Preheat oven to 350 degrees. Place a bowl in the refrigerator to get cold.
2. Combine graham cracker crumbs, butter, and 1/4 cup sugar in a bowl. Press mixture firmly over the bottom and up the sides of a 9-inch pie plate.
3. Bake crust in the preheated oven until firm, about 7 minutes. Let cool completely.
4. Whip cream in the cold bowl with a hand mixer until stiff peaks form.
5. In a separate bowl, stir sweetened condensed milk and frozen lemonade concentrate together. Fold lemonade mixture gently into the whipped cream. Pour filling into the cooled crust. Freeze until set, 8 hours to overnight.
6. Before serving, remove pie from freezer and allow pie to come to room temperature before slicing. In the meantime, toss the strawberries, blackberries, and raspberries with lemon juice and sugar in a bowl. Top each slice with mixed berries.
Enjoy!