Preheat the oven's broiler and set the oven rack about 6 inches from the heat source.
Place the grapefruit sections, orange sections, and pineapple chunks into an 8x8-inch baking dish. Stir the brown sugar and butter together in a small bowl, and scatter the butter mixture evenly across the fruit.
Broil the fruit until the sugar is bubbly, 5 to 6 minutes. Allow to cool for several minutes before serving warm.
