Put the flour, sugar and salt in the bowl of a food processor fitted with the metal blade and process for a few seconds to mix the dry ingredients.
Cut the butter into 1/2-inch pieces, drop them onto the flour, and pulse the machine 10 or 12 times, in short bursts, 20 seconds in all. The mixture should be crumbly with only a few larger bits of butter visible.
Sprinkle 3 tablespoons of water on top of the dough—immediately pulse about 6 times, only a second or two each time. You want the crumbs to gather together in wet clusters, a bit like cottage cheese curds—don’t expect a mass of dough to form. If they haven’t gathered, sprinkle on more water, a teaspoon at a time, and pulse a 2 or 3 times after each.
When the clusters form, scrape them all out of the bowl, press them together and knead just for a few seconds to form smooth, tight dough. Flatten it into a disk, wrap well in plastic wrap and refrigerate for 3 hours or up to a day before using. Freeze the dough for longer keeping.
(If the crumbs haven’t clustered after you’ve added 4 tablespoons of water, open the top and press them with your fingers—if they’re wet and stick together, just empty the bowl and press them into a disk of dough.)