Put the sugar into a medium stainless steel saucepan. Wash and dry the lemons, then finely grate the zest of 2 lemons, and add the zest to the sugar.
Squeeze all 3 lemons. Pour the juice into the saucepan with the sugar, add the soft butter in pieces and the lightly beaten eggs. Place the smaller saucepan into a larger one, partly filled with hot water (au bain marie/double boiler) and simmer until the cream begins to thicken. Stir constantly with a wooden spoon. Never allow the curd to boil! it should only simmer very gently.
Ladle the lemon curd into perfectly clean jam jars. Close the lids tightly and store in the fridge. Lemon curd is perishable because of the eggs, so don't store it for more then two weeks.