When I was a teen in Devon, I saw it done a few times when I visited the big city (Barnstaple!) I think that you needed parts from a Sturmey Archer three or four speed hub that had the built in dynohub or drum brake in order to get the spindle length.
Sturmey Archer still do a 3-Speed with Cassette hub. I had 3 x 8 on mine. It is currently in a 26" rim.
The 3-Speed is 75%/100%/133% as per standard. Its ok, but on an e-bike the pawls can slip under load.