Put the sugar and 1 cup water into a small saucepan and bring it to a boil. Cook for 1 minute until the sugar has dissolved, turn off the heat, and allow the sugar syrup to cool.
In a pitcher combine the cooled sugar syrup with the lemon juice and remaining 2 cups water and mix well. Fill a Champagne flute halfway with the lemonade and top with Champagne. Garnish with a lemon slice.