Trios

Whisk together flour and salt. Beat butter and sugar with an electric mixer until very pale and fluffy, about 4 minutes, then beat in egg and vanilla. At low speed, mix in flour mixture in 3 batches just until a dough forms. Divide dough in half and form each piece into a 6-inch disk, then chill, wrapped in plastic wrap, until firm, about 1 hour.
Preheat oven to 350°F with rack in middle. Line 2 large baking sheets with parchment paper.
Roll 3 separate level teaspoons of dough each into a ball, then flatten each ball slightly (to 1 inch wide and less than 1/2 inch thick). Arrange them in a triangle on baking sheet with edges touching in center, then make a deep indentation in center of each round with wooden spoon handle. Make more cookies, arranging them 1 inch apart on baking sheets.
Fill indentations in each cookie with about 1/88 teaspoon jam (each cookie should have 3 different fillings), avoiding any large pieces of fruit.
Bake until cookies are baked through and golden-brown on edges, 15 to 20 minutes. Cool on baking sheets 5 minutes, then transfer to racks to cool completely.
Bake more batches on cooled baking sheets lined with fresh parchment.