Preheat the oven to 375 F. Lightly spray a 9″ glass pie plate.
Place the pie crust into the base and form simple edging with your thumb and forefinger.
In a large mixing bowl, add the blueberries, raspberries and blackberries. Toss in the sugar, cornstarch and lemon juice. Pour into the pie plate and spread evenly.
In a small mixing bowl, whisk together the brown sugar, flour, cinnamon and salt. Cut in the butter with pastry blender or two knives, until crumbly.
Pour the crumbly topping over the top of the berries, and spread evenly to the edge of the pie plate, with a slight mound in the middle.
Bake in the preheated oven for 40 to 45 minutes, or until crumbly topping has turned brown and pie crust is baked through.
Cool for at least 30 minutes on cooling rack.
Slice and serve Berry Crumble Pie.