In a food processor, combine the sugar and the lemon zest until the zest is super finely minced amongst the sugar.
Juice all 3 lemons. If they do not yield 1/2 cup of juice, you can add water to make up for it, or if you have more lemons, you can juice them until you have enough.
If you have a doubler-boiler, you will want to use it for the next step. If not, a simple saucepan with a bowl placed on top with work just fine. Fill the bottom pan 1/3 of the way with water and bring to a boil. Reduce heat, but allow the water to continue to simmer.
While the water is coming to a boil , using the top pan of the double-boiler or in a medium bowl, whisk together eggs and sugar until well combined. Whisk in the lemon juice.
Place the pan or bowl over the pot with hot water, and whisk constantly, until the curd has thickened, about 8-10 minutes. You want to take this step slowly, to ensure you don't scramble your eggs.
Once the curd has thickened, remove it from the heat, and stir in your butter, whisking until the butter has completely melted.
Transfer the curd to a bowl, and cover with a piece of plastic wrap directly on the surface of the curd to prevent a film from developing.
Place in the refrigerator until completely chilled.
Use the curd for pie, spread it on toast, or eat it by the spoonful.
Lemon curd will keep well in the refrigerator for up to one week.