Cover a baking sheet with parchment paper. Draw 6 (4-inch) circles on paper. Turn paper over; secure with masking tape. Set aside.
Beat egg whites at high speed of a mixer until foamy. Gradually add 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. (Do not underbeat.) Fold in pecans. Divide egg mixture evenly among 6 drawn circles; using the back of a spoon, spread egg white mixture onto the circles.
Bake at 275° for 2 hours or until dry. Turn oven off; let meringues cool in closed oven at least 12 hours. Carefully remove meringues from paper; set aside.
Combine peaches and 1/4 cup sugar; stir well. Let stand 30 minutes. Combine 1/4 cup sugar, raspberries, water, and lemon juice in a saucepan; bring to a boil. Remove from heat. Place raspberry mixture in a blender; process until smooth. Strain puréed mixture through a sieve, discarding seeds. Let cool.
Spoon 1/4 cup raspberry sauce onto each of 6 dessert plates, and top with meringues. Arrange 2/3 cup peach mixture and 1/3 cup blackberries in each meringue; serve immediately.