In a mixing bowl, combine strawberries, blueberries, and raspberries. Add granulated sugar, lemon juice, cornstarch, and vanilla extract. Toss gently to coat the berries evenly.
Pour the berry mixture into a greased 9-inch baking dish, spreading it out evenly.
In another bowl, mix together the rolled oats, all-purpose flour, brown sugar, cinnamon, melted butter, and a pinch of salt until crumbly.
Evenly spread the oat mixture over the berry layer in the baking dish.
Bake in the preheated oven for 25-30 minutes, or until the crumble is golden brown and the berries are bubbling.
Remove from the oven and let cool for a few minutes before serving.
Notes
Serve warm with a scoop of vanilla ice cream or whipped cream on top and garnish with a sprig of mint for an extra pop of color.