This game, known to most card game books as Yablon, is also commonly called Ace-Deuce, Acey-Deucey, In Between or Between the Sheets. American casinos nowadays call it Red Dog, which is confusing, since since Red Dog was formerly the name of a different game.
Yablon is a banking game in which two cards are dealt face up, and the player bets on whether a third card will rank between the first two cards. Detailed rules can be found in the FAQ of the newsgroup rec.gambling.misc, where it is listed under the name Red Dog.
The Red Dog page at The Pogg analyses the different variants of this game available online and offers optimal strategies.
Roland Scheicher has written an article on Red Dog for the German Wikipedia.