Put the blueberries, peaches and raspberries in a 2-quart baking dish and roast until soft, 10 to 12 minutes. Strain the fruit juices into a saucepan and return the fruit to the baking dish. Bring the juices to a simmer over medium heat and reduce until syrupy, 10 to 12 minutes. Pour over the fruit.
For the crumble: Reduce the oven to 325 degrees F.
Combine the oats, brown sugar and melted butter in a small mixing bowl. Spread the crumble mixture on a sheet tray and bake until it just starts to brown, 10 to 12 minutes.
Sprinkle the crumble on top of the fruit. Scoop to serve with Greek yogurt.