Why do people keep repeating this mis-information? Are they Shimano employees?
Sturmey introduced the improved AW-NIG twenty-five years ago. NIG is an acronym for "no in-between gear"; there is no neutrals between the in-gear positions. The -NIG versions of the hub are the only ones ever produced by SunRace Sturmey-Archer (that's every Sturmey AW-family hub produced in this millenium.)
tcs
i'm aware of S-A's introduction of the NIG hub (i mentioned i had one in an earlier post to this thread), but as the poster above me just alluded, the vast majority of vintage 3-speeds still floating around, at least in the US, predate the NIG hub.
so, my comment is perfectly valid and bears repeating here-- the "pre-NIG" AWs will have a tendency to drop into freewheel under sustained heavy loading of the clutch in top gear-- even if the spindle is in perfect adjustment. if you disassemble an old AW and examine how third gear works, the flaw becomes evident.