Finely grate one lemon to obtain about 1 tablespoon lemon zest; set zest aside. Juice all 3 lemons; set juice aside.
Place the gelatine sheets in a bowl with a little water, so that they soften.
Separate the egg whites from the yolks. Mix egg yolks with sugar until smooth, about 3 minutes. Pour into a saucepan over low heat, gradually add the lemon juice and lemon zest; cook until the mixture thickens. Remove from heat and let cool.
Drain and squeeze the gelatine leaves, then add to the sugar-egg-lemon mixture.
Add a pinch of salt to the egg whites before beating them until they are firm.
Gently stir the egg whites into the lemon mixture.
Pour into individual ramekins; place in the fridge and chill the mousse for at least 2 hours to 3 hours before serving.