For crust: stir together first three ingredients. Cut butter into flour mixture. Whisk together egg yolk and milk. Stir into butter/flour mixture. Shape into a flat disc, wrap in plastic wrap, and chill at least 1 hour.
For filling: Preheat oven to 425 degrees. Mix peaches, plums and blueberries in a bowl. Stir together sugar and flour. Stir into peach mixture. Pour fruit into a 9x13 inch pan. Drizzle with melted butter.
Roll dough for crust to 1/4 inch thickness. Cut into 2 inch squares. Lay squares in a patchwork pattern over peach mixture. Whisk together egg and water. Brush over crust. Sprinkle with sanding sugar if desired.