For pastry:
Combine flour, sugar and salt in processor. Add butter; using on/off turns, cut in until mixture resembles coarse meal. Add ¼ cup ice water and process using on/off turns until moist clumps form. Gather dough together. Divide dough into 6 equal pieces. Flatten into disks. Wrap each in plastic. Chill until firm, at least 1 hour or overnight.
Position rack in center of oven and preheat to 400°F. Roll out each dough disk on floured surface to 5 ½-inch round. Transfer rounds to 4 ½-inch-diameter tartlet pans with removable bottoms. Press dough onto bottoms and up sides of pans. Place pans on baking sheet.

For filling:
Toss nectarines, plums and sugar in large bowl. Arrange fruit in crusts. Bake tartlets 10 minutes. Reduce heat to 350°F and continue baking until fruit is tender and beginning to bubble and crusts are golden, about 40 minutes longer. Transfer tartlets to rack and cool completely. Place berry in center of each.