You have pictures in your gallery. Here is one of your hub:
I can't see what hub that is, nor could I say what thread that axle is off the top of my head. Somebody will be along shortly. Here is a link to your gallery:
https://www.bikeforums.net/g/album/25278991 There is also a dedicated three speed thread here:
https://www.bikeforums.net/classic-v...-3-speeds.html where you could ask.