With a Sturmey, just use the factory DT shifter, as several have suggested.
With the Shimano, well, first, you gotta hit second gear position accurately or your 3-spd hub will have a short and unhappy life. Okay, so there's a second gear alignment mark and the other two gears are found at cable slack and cable taut. (Internet semantic debate about definition of 'taut' goes here.) You could use a friction shifter and hit second gear by feel or estimated position, like Sheldon Brown did. More sure would be to use a many speed indexed DT shifter, and set one of the middle detents (and you'll have to remember which one!) to second gear alignment.