Place the rhubarb, water, and 1 cup of sugar in a sauce pan. Mix and bring to a boil over medium heat. Once it starts boiling, reduce heat to medium low so it simmers but doesn't burn. The sugar should be melting, but you can also whisk to make sure it is all mixed together.
As the rhubarb gets soft, add the strawberries and blueberries. Cook for another few minutes until there are no more big pieces of any of the fruit.
Add the 4 Tablespoons of flour and whisk until combined. Heat for another minute or two until the filling is thicker, almost like jam.
Gently pour the rhubarb berry filling over your uncooked crumble base. Smoosh around with a rubber spatula until it's in an even layer.
Top with the remaining crumble then place in the oven.
Bake 35-40 minutes until the top layer is browned and the filling is bubbling.
Remove from oven and let cool in pan for at least 1 hour. Then, slice into pieces and enjoy!