Chop one peach. Add to a small saucepan with water and sugar. Bring to a low boil over medium high heat, stirring and crushing peaches with the back of your spoon.
Reduce heat to medium and simmer until syrup has thickened slightly, about 12 minutes. Remove from heat and allow to cool.
Chop one peach, and slice remaining 2 peaches.
Add chopped peaches and a small handful of raspberries to a pitcher or large wide mouth mason jar. Muddle fruit.
Add sliced peaches, remaining raspberries, and Riesling. Sweeten to taste with peach syrup. Serve over ice and top with sparkling wine, if desired.