What is the significance of George Bernard Shaw's Arms and the Man being written in three acts instead of the usual five?

The division of plays into “acts” has three functions: the social need for pauses in the entertainment experience, both physical and psychological; the theatrical necessities of costume change, set change, actor breaks, and the like; and, most importantly, the division of the “narrative” into parts whose relationship is part of the artistic experience.  It is not quite accurate to say that plays are “usually” divided into five acts; history has shown that the stage has favored three acts ever since the 19th century, and ever since the 20th century the two-act play is more common.  The important difference is what the playwright was trying to do: the five-act play is a full historical account, as in Shakespeare’s English histories.  The three-act play subtly follows the Socratic method of argument, with a thesis (Act I), an anti-thesis (Act II) and a conclusion (Act III).  Arms and the Man follows this pattern.  In modern times, starting with Beckett’s Waiting for Godot, playwrights have found the 2-act play structure best, to illustrate the dualities presented by Existentialism and other dualistic views of the world.  The whole argument is complicated and requires study in both philosophy and dramaturgy.