According to "the book," what is the actual, although unstated, purpose of endemic war between the three superstates?
This can be answered in one word: POWER. Maybe another will work too: CONTROL.
It seems as if the three states have similar goals in controlling their people and the need for government is derived from the need to be kept safe from the other peoples. Although stated purposes might be using up the supplies produced by the peoples, the truth is this constancy of war gives people a nationalism and a need for those that lead them. The fact that the people don't understand WHY they are at war doesn't matter, what matters is their allegiance.
The three superstates must have chosen this path because history, true history, not the created one they alter daily, has demonstrated that governments ultimately get soft and lose power. Oceania obviously didn't want this to happen.