Mix the sugar, flour, and salt in a separate saucepan and then slowly stir in the hot milk. Cook until thickened, stirring regularly, over medium heat.
The three egg yolks should be slightly beaten and ready in a small bowl. Add a small quantity of the hot mixture to the beaten yolks and whisk until well blended. Finally, add the yolks back into the hot liquid.
If preferred, you can either let the pie cool before serving, or you can top it with a meringue (you'll have three egg whites left over).