Bring rhubarb and 3 cups water to a boil in a large saucepan. Cook until the rhubarb is very soft and broken down, about 5 minutes. Transfer to a medium bowl. Put a couple inches of ice water in a large bowl and set the bowl with the rhubarb in it to help cool it quickly. (If you aren't in a hurry, you can skip the ice-water bath.) Refrigerate, stirring occasionally, until cool, at least 20 minutes.