Combine flour, sugar, and salt in a food processor.

Add chilled butter and pulse until the mixture resembles coarse meal.

Add ice water and pulse until moist clumps form.

Gather the dough together and divide it into 6 equal pieces.

Flatten each piece into a disk and wrap in plastic wrap.

Chill the dough in the refrigerator for at least 1 hour or overnight.

Preheat the oven to 400F (200C) and position a rack in the center.

Roll out each dough disk on a floured surface into a 5 1/2-inch round.

Transfer the dough rounds to 4 1/2-inch tartlet pans with removable bottoms.

Press the dough onto the bottoms and up the sides of the pans.

Place the tartlet pans on a baking sheet.

Toss nectarines, plums, and sugar in a large bowl.

Arrange the fruit in the tart crusts.

Bake the tartlets for 10 minutes.

Reduce the oven heat to 350F (175C) and continue baking until the fruit is tender and beginning to bubble and the crusts are golden, about 40 minutes longer.

Transfer the tartlets to a wire rack and cool completely.

Place a blackberry in the center of each tartlet.

Stir plum jelly in a heavy small saucepan over low heat until melted.

Brush the warm jelly over the tartlets.

Let stand at room temperature.