For the graham cracker crust, preheat the oven to 350. Crush the graham crackers in a bag (make sure its sealed!) with a rolling pin. This can also be done in a food processor if you have one. once the graham crackers are crumbs (make sure no large pieces remain), mix the crumbs, melted butter and sugar in a medium bowl. Press the mixture into a pie pan (I use a 9.5 in pan but smaller will work as well). Form the edge as you please and then bake until golden brown, about 7 minutes. Set the pie crust aside, don't worry if it doesn't seem entirely set, once it cools it will be perfect! The pie also bakes at this temperature so you can leave the oven at 350.
For the pie filling, using an electric mixer at high speed, beat the 3 egg yolks and lime zest until fluffy (about 5 minutes). Slowly add the sweetened, condensed milk and beat until thick (3-4 minutes). Lower the speed of the mixer and gradually add the lime juice, mixing until combined but no longer. This is also when you can add the green food coloring if you want the pie filling to be lime green (add one or two drops util you get the color you want). Pour the filling into the cooled pie crust and bake at 350 until set, about 10 minutes. Allow the pie to cool on a wire rack and then refrigerate.
Before serving whip the heavy whipping cream and maple syrup (add to taste) in a medium bowl. It works best if the bowl is chilled, so I usually place a metal mixing bowl in the freezer shortly before hand. Decorate the pie with whip cream and serve!