Mix 2 cups rhubarb and 1 cup sugar together in a bowl; set aside to allow juices to release, about 10 minutes. Combine flour, sour cream, baking powder, and salt in a bowl. Add rhubarb-sugar mixture to flour mixture; mix until just blended. Pour into an 8-inch square baking pan.
Bake in the preheated oven until golden brown, 25 to 35 minutes. Cut shortcake into 9 squares.
Mix 1 1/2 cups sugar and strawberries together in a bowl; set aside to allow flavors to meld, about 10 minutes.
Place 3 cups rhubarb and water in a saucepan over medium heat. Stir in 1 cup sugar; bring to a boil. Lower heat; simmer until tender, about 15 minutes. Remove from heat; mash with a potato masher. Return to medium heat; add cornstarch and return to boiling. Simmer, stirring, until sauce is thick, 3 to 5 minutes.
Pour strawberry-sugar mixture into rhubarb sauce; stir until well mixed. Cool completely in the refrigerator, about 1 hour. Spoon sauce over shortcake squares.