The classic reason for that fault in an old 3-speed SA is minor misadjustment of the gear change cable. The gear lever basically pulls a chain further and furth

The classic reason for that fault in an old 3-speed SA is minor misadjustment of the gear change cable. The gear lever basically pulls a chain further and further out of the hub, going from 3 at minimum pull to 2 to 1. Unfortunately there's a "neutral" between 3 and 2, so the cable needs to be carefully adjusted so that the 3/2/1 positions of the lever give you all three gears and avoid the neutral section. This design defect was apparently rectified in later versions of the 3-speed hub at some point after SA were bought by Sun Race. Still, I wouldn't personally ever stand up to pedal on an SA 3-speed unless I personally owned it and knew its maintenance condition...