Heat oven to 180C/160C fan/gas 4. Line the base and sides of a shallow rectangular tin (about 23 x 33cm) with baking parchment, snipping the corners so that it comes up the sides of the tin. Tip the flour, sugar and butter into a food processor and whizz until it forms fine crumbs. Tip into the prepared tin and smoothly level with the back of a metal spoon, pressing it down lightly. Bake for 20-25 mins until pale golden.
To make the topping, finely grate the zest from the lemons and orange and squeeze out all the juice. Measure the juice - you need 120ml. Whisk together the eggs and sugar using an electric whisk for 1 min, then add the citrus zest and juices, and whisk again briefly. Sift in the flour and whisk well to mix. Pour over the shortbread and bake for 15-20 mins until the topping has just set. Cool completely in the tin, then lift out of the tin using the lining paper. Dust thickly with icing sugar and cut into 3 down the length and 6 across to make 18 bars.