Rinse, dry and zest one of the lemon. Slice and squeeze the zested lemon plus one other and measure 3 ounces (90ml) of lemon juice. If needed, use the third lemon.
Combine the zest, juice and sugar in a small sauce pan and stir until blended. Cook over medium-low heat, until the sugar has completely dissolved, stirring constantly. Liquid should be transparent. Be sure to scrape down any sugar that collects on the side of the pan. Remove from the heat and cover to keep warm.
In a heavy medium sauce pan, gently bring the cream to a boil over a low heat stirring frequently to avoiding scalding and skimming. Allow to simmer for 3-4 minutes string occasionally.
Remove from heat, and gently whisk the lemon syrup into the cream until well blended.
Using a fine mesh strainer, strain the cream mixture into a bowl or glass measuring cup with enough capacity for 3 cups.
Using a ladle or the spout of the measuring cup, divide the posset evenly into four custard dishes. Allow to cool on the countertop for 20-30 minutes, then refrigerate for 2-4 hours until firmly set.
For longer storage, cover custard dishes with plastic wrap. Lemon posset is best if eaten within 48 hours.