Rub the peel of the oranges and lemon. Squeeze the oranges and lemon. Put the sugar, grated zest, juice and citric acid in a large container, mix together. Pour hot water over it and stir until the sugar has dissolved. Close with a lid. Let stand for 5 days, stirring the juice every day.
Strain and fill into bottles. The syrup is drunk diluted with tap or mineral water.