Preheat oven to 400 degrees F (200 degrees C). Lightly grease a 9x13-inch baking dish.
Pour pears, peaches, and cherries with their liquids into the prepared baking dish; stir. Stir oats and 1/4 cup brown sugar together in a bowl; sprinkle over the fruit mixture. Sprinkle remaining 1/4 cup brown sugar over the oat layer. Sprinkle cinnamon over the brown sugar. Arrange butter pieces evenly over the top.
Bake until hot, bubbly, and slightly brown at edges, about 20 minutes.
