There are plenty of new 3 speed hubs available. I don't know if there's a kit. I believe you'd have to measure your rim and hub and use an online spoke calculator to tell you what spokes to buy.
You can also get new 3 speed wheels.
Of course, you can also buy new 5 speed hubs from Sturmey-Archer and SRAM.
Your frame can be respaced to a different width hub. I don't know the widths of the various hubs except for a Shimano Nexus 3 Coaster Brake which is 120mm.
The chainline is another issue you would have to get right. I don't know the chainline of various new hubs.