Beat yolks in a very large bowl until thick and pale. Slowly beat in sugar. Whisk in milk and 2 cups cream. Mix in bourbon, rum, and Cognac. Cover, and refrigerate for up to 1 day.
Just before serving, beat whites until stiff peaks form. Fold whites into eggnog. Whisk remaining 1 cup cream until stiff peaks form, and fold into eggnog. (Alternatively, you can fold half the whipped cream into eggnog, and top with remaining half.) Sprinkle with nutmeg.