In the other version of A&A you have players run multiple nations.
Two players: one player gets the Allies the other gets the Central Powers.
With a three player game I would have one player take the Central Powers, one take Russia and the last player take the Western Allies.
Four players will probably split the Central Powers with one taking Germany and A-H and Turkey for the second player.
I would think that unless 8 players are playing, the person playing Russia also plays the US