Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch pan.
Bring water to a boil in a small saucepan. Dissolve 1 cup sugar in boiling water, then add 3 tablespoons butter. Remove from heat, and set syrup aside to cool.
In a large bowl, whisk together flour, baking powder, and salt. Cut in 1/2 cup butter. Stir in milk; mix lightly. Roll out on a floured surface into a rectangle shape about 12 inches long.
In another bowl, combine rhubarb, cinnamon, and 1 cup sugar. Spread evenly over roll, leaving a 1/2 inch edge. Roll dough tightly lengthwise, and seal the seam. Cut into twelve 1 inch sections. Arrange rolls in prepared pan. Pour syrup over rolls.
Bake in preheated oven for 40 minutes, or until lightly browned.
