Preheat oven to 400 degrees F (200 degrees C). Butter a 2 1/2-quart baking dish.
Stir rhubarb, blueberries, strawberries, lemon juice, pearl tapioca, and 1 cup raw sugar in a bowl. Transfer to prepared baking dish.
Place butter into a separate bowl and chop into small pieces with a pastry cutter. Cut in flour, 2 tablespoons raw sugar, salt, and cinnamon; stir in water, 1 tablespoon at a time, until the dough holds together. Drop small pieces of the dough on the fruit.
Bake in the preheated oven until the filling is bubbly and the cobbler is lightly browned, 40 to 50 minutes. Allow to stand 10 minutes before serving.