Add a few inches of water to a medium saucepan, or the bottom pot of a double boiler. Bring the water to a boil over high heat, then reduce to medium heat to keep the water at a simmer.
While the water is coming to a boil, zesting all 3 lemons.
In a food processor, combine the sugar and the lemon zest until the zest is super finely minced amongst the sugar. You can also mix by hand.
Juice all 3 lemons, or more if needed, to get to 1/2 cup of juice.
In the top pan of the double-boiler or in a medium heat-proof bowl that will fit on top of the saucepan, whisk together eggs and sugar until well combined.
Whisk in lemon juice and salt.
Place the pan or bowl over the pot with hot water, and whisk constantly, until the curd has thickened, about 10-15 minutes. You want to take this step slowly, to ensure you don't scramble your eggs.
Once the curd has thickened, and reaches a temperature of 170F, remove it from the heat, and stir in your butter, whisking until the butter has completely melted.
Optional step, but it can help with texture: pour the curd through a fine mesh sieve into a clean bowl. This will catch any eggs that may have scrambled. Make sure to scrape the underside of the sieve afterwards.
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. The curd will thicken further while chilling.
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.