Generously grease hands and counter with some of the shortening. Turn marshmallow mixture onto counter. Sprinkle 1 cup of the powdered sugar over marshmallow mixture and knead as you would dough. Continue to add the remaining 2 1/4 cups powdered sugar, greasing hands and counter often to prevent sticking. If fondant begins to tear, it is too dry; add remaining water, 1/2 teaspoon at a time, and knead until fondant forms a firm, smooth, elastic ball that will stretch without tearing, about 8 minutes. Wrap fondant in a double thickness of plastic wrap, squeezing out any air; set aside.