Spray the slow cooker with non-stick cooking spray.
Add the raspberries, blackberries, strawberries, sugar, cornstarch, and preserves to the slow cooker. Stir until all the berries are coated evenly.
In a small bowl, combine the oats, brown sugar, flour, butter, and cinnamon. Use a fork or pastry blender to cut the butter into the mixture.
Sprinkle the oat mixture over the berries.
Place the lid on and set the slow cooker to high. Cook for 2 1/2 hours. Berries will be bubbly and oat topping will be a little firm.
Optional: Turn your oven broiler on low and remove the lid to the slow cooker. Place the slow cooker insertin the oven for 2 - 3 minutes until top is lightly browned. Remove from oven.
Allow to cool for 15 minutes before serving.