I have! they're weird in there. You can pick up a rear hub for short money on ebay -- refurbishing one is a good way to spend a week or so, and Saint Sheldon has all the details: https://www.sheldonbrown.com/sturmey-archer_3-spd.html

https://www.sheldonbrown.com/sturmey-archer/aw.html has teardown photos, to boot.