The Shimano Nexus 3 speed does not require much maintenance as it is encased in a sealed unit. However if there are problems in changing gears, the gear shift system can be adjusted.
To adjust, select the middle gear (2nd gear) on the controller then loosen the lock nut on the gear, and turn the adjustment knob.
Ensure that the yellow indicator is positioned between the two white lines on the display window. Remember to re-tighten lock nut to set.