This has nothing to do with little AAA batteries, of course.
The entire improvisation is in the key of A, and played on two manuals and pedal, therefore the three A's in the title, and it keeps coming back to these A's, even when modulation happens.
It stats with reeds 16-8-4 on the Hauptwerk plus pedal 32' flue and 16' reed, and then builds up from there.