A3 is simply a reference to the size of an 11x17 piece of paper.
An A3 report is a way of organizing and analyzing issues that requires the team to implement the discipline of the scientific method represented by the Attend, Plan, Act, Learn, Improve Cycle.
This system rigorously distills the background, the problem, the current state, the future desired state and the proposed counter-measures to get to the future state all on a single, 11” by 17” piece of paper.
The purpose of the A3 process is to structure efficient and effective dialogue, in a way that creates the opportunity for deep understanding and agreement. The resulting genuine communication and dialogue leads to better decisions, in which the proposed countermeasures are more likely to succeed, because they are based on Evidence gathered from the people actually performing the work.