Boil 700 ml milk. Stir pudding powder, 3 tbsp. sugar and 100 ml milk until smooth. Stir into the boiling milk, bring to the boil and simmer for about 1 minute. Remove from the stove. Place in a bowl and cover directly on the surface with foil.
Let cool down at room temperature.
Knead 500 g of sugar, butter and flour into crumbles. Select raspberries, blueberries and blackberries. Wash blueberries if necessary and drain well. Wash the strawberries, drain well and clean.
Spread approx. 2/3 of the crumble as a base on a greased, flour-strewn baking tray (34 x 40 cm) and press on. Stir the pudding briefly with a whisk, place on the crumble base and smooth it down.
Spread the berries on the pudding and crumble the rest of the crumbles over it. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 50 minutes, remove from oven and let cool on a cake rack.