Tinnishill wrote: ….I do have this niggling doubt, however, about the old Sturmey Archer injunction to never exceed a two to one ratio between chain wheel and drive sprocket....
where might this be writ, exactly? FWIW in old SA 3s hubs the usual limiting factor in hard use was that the sun pinion was riveted to the axle. [Modern SA hubs have one-piece axles which are an inherently stronger design; indeed you can upgrade an older SA AW hub with a new style, stronger axle. ]
re CS-RF3; I have seen one or two of these hubs blow up, and the cause of the blow-up appears to have been a crap gear cable, that wouldn't allow the hub to shift itself all the way into third gear. This resulted in the high gear clutch slipping and this wrecked the hub. (*)
The 'window' for second gear to be safely selected is quite wide, so you are unlikely to come to grief this way; IME the most likely time for a 'distress shift' is when selecting the low gear in the hub, which again has a very wide 'window'. [In essence the hub internals have been modified to allow a MTB RH shifter to be used, which has a much longer cable pull than is 'normal' for a 3s hub. The result is that gear 2 and gear 1 have a wider 'window' than in many other 3s hubs.]
Some models of shimano LH thumbshifter/bar end shifter (which are otherwise friction shifters) have a tactile detent in the middle position; one of these would be ideal for this hub, I would have said. However I don't think I've ever seen a list of shifters that have this feature; I don't think any of the current models do.
(*) if the high gear clutch -which has sharp edges- gets chipped, the fragments stand every chance of jamming the plunger pawls in the planet cage. To my mind this means if the hub slips in high gear, even once, then it should be stripped and cleaned, else reliable operation subsequently cannot be guaranteed.