Originally Posted by BromptonINrio

for 3 speed hub there is trhee easy opitons:
1) you can keep 3 speed hub and buy a new 28h rim:

349 bike rim double layer rims 161-3/8 aluminum alloy bicycle rim 16/20/24/28/36 hole 16 inch cyling rim
https://s.click.aliexpress.com/e/b5wXpb0G

2) you can buy a new hub and a new rim, either 5 or 8 speed from sturmey archer:
5 speed here( 28h rim)
​Sturmey archer SRF5(W) Internally Geared 5 speeds bike bicycle rear Hub 28H 110mm with shifter lever set fit for folding bike
https://s.click.aliexpress.com/e/BojDqfe

8 speed Sturmey archer (36h rim)
​​​​​SUNRACE X-RF8 V brake within 8 speed hub 36 h
https://s.click.aliexpress.com/e/b9JIRWQc

3) you can buy a new rim and for gearing, you can add in crankset a internal geared crankset, like fsa metropolis(cheaper), or schulmpf(expensive):

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F264115804410