I suspect that between here in Poland and the other side of Asia there are tons of bikes running 3 speed derailleur, mine is a Falcon and you can see it was built on a hub that could easily fit 5. You could choose just the sprockets you wanted.


3-speed Falcon freewheel

While on AliExpress I came across a 3 speed derailleur built onto what looks like a single-speed freewheel hub.