Preheat the oven to 325 degrees F (165 degrees C). Combine graham cracker crumbs, white sugar, and melted butter in a medium bowl. Press evenly into a 9-inch pie plate.

Bake the crust in the preheated oven until lightly set, about 5 minutes. Let cool.

Separate 2 eggs, placing the egg whites in a large bowl and the yolks in another bowl. Add the remaining whole egg to the yolks. Whisk in key lime juice and sweetened condensed milk until smooth.

Beat egg whites with salt and cream of tartar until stiff peaks form. Gently fold into the lime mixture until no streaks remain. Pour filling into the cooled crust.

Bake until the filling is just set, 10 to 15 minutes. Let cool to room temperature.

Freeze until firm, at least 4 hours or overnight.

Beat heavy whipping cream until stiff peaks form. Garnish the pie with whipped cream and lime slices before serving.