Combine the red wine and 1/3 cup sugar in a medium saucepan. Bring to a boil over medium-high heat, then reduce to a simmer and continue to cook until the liquid is reduced to about 1/4 cup, 10 to 15 minutes. Let cool completely.
Combine the cool syrup with the blackberries, cherries, strawberries and nectarines in a large bowl. Let marinate, refrigerated, until the fruit starts to macerate and release juices, about 1 hour and up to 4 hours.
Preheat the oven to 400 degrees F.
Whisk to combine the flour, baking powder, salt and remaining 3 tablespoons sugar in a large bowl. Add the heavy cream and mix until combined.
Press the dough into an 8-inch square baking pan and bake until the edges are golden and the shortcake is cooked through, 18 to 20 minutes. Transfer the pan to a wire rack to cool completely.
Cut into 6 pieces and split each shortcake in half horizontally. Spoon fruit on the bottom of each shortcake and top with a generous spoonful of whipped cream. Place the top of each shortcake over the whipped cream, then top with the remaining fruit. Drizzle the fruit sauce over the fruit and garnish each with a dollop of whipped cream and a mint sprig.