Grate outside rind of two lemons; add sugar, flour, and cornstarch. Stir in beaten egg yolks, then water and lemon juice. Over medium heat, stir constantly till the mixture is thickened. Stir in butter. Pour into pie shell.
Prepare Meringue: Beat egg whites until foamy; stir in 3 tablespoons sugar and continue beating until stiff. Carefully spoon onto pie. Bake at 425 to 450 degrees F. for 3 to 5 minutes, or till meringue is lightly browned.