Is it a Shiumano Nexus 3 speed? If so, it is very easy to adjust. put the shifter in 2nd gear, then on the axle part of the shifter (bellcrank), turn the barrel adjuster screw until the red mark in the window lines up between the 2 lines. That's it!

If it is a (Sturmey Archer (Sun Race), place in 2nd gear, rotate the pedal to settle the gears, adjust the barrel screw until the pull rod at the end of the mini chain is flush with the axle (you will probably need to pull off a plastic protective cover to do this)

Both these adjustments are on YouTube videos.

Don't be annoyed if a barrel adjustment is out of whack. Even the most prestigious of bikes must deal with these adjustments. It is something the rider needs to know how to do. Cables stretch, and housings settle. They must also be readjusted every time the rear wheel is removed.