This information will help you adjust your Sturmey Archer 3-Speed Hub. However, if you can get to a bike shop, we highly recommend doing that to have it professionally adjusted.
STURMEY ARCHER INFO:
How to adjust a Sturmey Archer 3 Speed Hub:
How does a Sturmey Archer Hub work? (Informational - No adjustment info here.)
The links below apply whether you have a newer style hub with the large plastic housing that covers a plastic roller/pulley cable router, or if you have the older style that has a metal elongated axle nut with a hole at the end that a shifter chain runs through.
Instructions from Sturmey Archer