Internal gear hub maintenance
My experience as a bicycle mechanic in the Netherlands: don't mess with it.

If it is an Shimano, it will probably run >100.000 km. If it is a Sturmey Archer, SRAM or any other well known brand, it will mostly do >50.000 km as well. 

If there is a small hole for lubricant, only do this once a year or so, with some thin grease, but not oil. Oil is to thin. 

If it doesn't shift well, first look at the cable. You probably only need to adjust the tightness of the cable. If this doesn't work, de-attach the cable. Every hub must work correct in the smalles or biggest gear without a cable attached. If this still doesn't work, the hub is broken. (Happens almost never.) If a 3-gear hub is broken, it is easy to take apart and build together again. And finding which gear or bearing is broken isn't difficult either. Finding a bearing balls is easy to, but finding a spare gear is probably not so easy.

Main point: leave it as it is.