In a 12-inch circular baking dish or deep dish pie pan, toss the mango chunks, apple pieces and raspberries. Squeeze the juice from one lemon over the fruit and toss. Sprinkle the brown sugar and 3 Tbsp of flour over the fruit and stir it together.
In a mixing bowl, whisk together the remaining 1 cup flour, sugar, oats and salt until well combined. Cut in the butter using a pastry blender, fork or two knives, until pea-sized pieces form. Spread the topping over the fruit in the baking dish.
Bake at 375 F for 45 to 50 minutes, or until crust begins to golden on top.
Serve warm or chilled as is, or warm with ice cream.