Preheat the oven to 375°F (190°C). Lightly grease the bottom of a 9x13-inch casserole or ceramic baking dish with butter.
Add the flour, rolled oats, sugar, cinnamon, and salt to a mixing bowl. Whisk until evenly combined and free of lumps.
Add the cold cubed butter. Rub the butter into the dry ingredients with your fingertips until the mixture resembles large, coarse breadcrumbs with some crumbly pieces remaining.
Add the mixed berries directly to the prepared baking dish. If using particularly large berries, roughly chop them first if desired.
Add the cornstarch, lemon juice, and a pinch of salt to the berries. Gently toss until the fruit is evenly coated.
Scatter the oat mixture evenly over the berries, covering the fruit without pressing the topping down firmly.
Bake the Triple Berry Crumble With Oats for 40–45 minutes, or until the fruit is bubbling and the oat crumble topping is crisp and golden brown.
Remove from the oven and allow the crumble to rest briefly before serving.
Serve warm on its own or with vanilla ice cream or cold whipped cream.