Stir together flour, granulated sugar, and salt, then add beer, and whisk to combine. Heat oil in a 5-quart heavy pot until the thermometer registers 375 degrees F.
Dip apple slices in batter, shaking off excess, and fry, about 8 at a time, until golden, about 1 1/2 minutes on each side. Transfer fritters with a slotted spoon to paper towels to drain. Serve warm, dusted with confectioners' sugar.