So long as we're talking about 3-speeds, I encourage folks not to be intimidated and do the job per factory rather than sling seeping oil all over their rear wheel. Maybe that's just me. Anyway, SRAM (Fichtel&Sachs) and Shimano 3-speeds come apart with a cone wrench and std. open-end wrenches. Sturmey & Sturmey copy 3-speeds can be tapped apart with a drift punch. There are lots of manuals, guides and videos on this.
Good to know, thanks. I have a folder with a SRAM 3-speed IGH, bought it from a grad student leaving town (never used it, didn't know the buses here had racks for his mountain bike). Owners manual says "permanently lubricated" and no oiling port, but I have considered dripping gear lub in the shifter chain hole. Just hasn't been urgent, I instantly discovered the gear range was not wide enough for me, but I don't want it ridden a lot until I know it is well lubed. I was able to print out the factory service manual at the library.