Preheat oven to 350 degrees F. Grease a 13x9-inch pan. For crust, combine the 2 cups flour, brown sugar and cinnamon. Cut in the 1 cup unsalted butter until crumbs form. Press into bottom of pan. Bake for 15 minutes.
For the filling, blend eggs, sugar, the 1/2 cup flour and salt. Fold in rhubarb, strawberries and blueberries. Carefully spread onto crust. For the crumb topping, combine the melted 1/4 cup butter, 1/2 cup flour, brown sugar, oats and cinnamon. Sprinkle the crumb topping on top of the rhubarb mixture. Bake for 45 to 50 minutes, until lightly brown and filling has set.