S3X is not an especially good hub IMHO. It has a lot of lash (so it isn't a very good fixed gear) and the internal it is based on the single toggle (W) series of 5s hubs; these hubs are very sensitive to adjustment variations and internal faults are more common than they should be. There is a small amount of parasitic drag with this hub even in the direct drive gear. SA discontinued both the single toggle (W) 5s hubs and the S3X several years ago. For now, spares are available, but they won't be for ever.
On the plus side the 'high' ratio in the S3X is direct drive, which is the way you might choose to have it.
FWIW I'd choose either a current S-RF3 (NIG) three speed gear, or a later(current) aluminium shell (eg from S-RF3) with an earlier (pre NIG) AW internal installed inside it. A pre-NIG internal can be fitted with a new 'all solid' axle which should be stronger than the old sort with a riveted sun pinion. There is a whole raft of small changes which can be made to these hubs which make them likely to last longer without giving trouble, many using standard SA parts which are readily available.
However I would not choose the rotary shifting RS-RF3 hub; not because it is weak internally (although it might be for all I know) but because the shift mechanism takes up about 14mm on the RHS of the axle which means that the wheel goes from 'barely dished' to 'as dished as a derailleur wheel', thus negating one of the basic advantages of an IGH. It won't even fit into narrow-spaced frames at all.
One of the reasons for selecting the pre NIG 3s internal is that the driver is long enough to accommodate two sprockets. One option for setting the hub up is to have a 'low range' (which uses the larger sprocket) which you can select for 'climbing days'. One method of doing this is to carry a few spare links of chain and a another master link, so that the chain can be lengthened and the larger of the two sprockets used. This works most easily if the difference in the sprocket sizes is a multiple of four; this way the chain needs to be lengthened by an even number of links, often without moving the rear wheel. Other teeth differences are also possible if you use a half-link.
For example without a half link
4 teeth difference, two extra chain links
8 teeth difference, four extra chain links
12 teeth difference, six extra chain links
With a half link
two teeth difference, one extra chain link
six teeth difference, three extra chain links
ten teeth difference, five extra chain links
All this works more easily if you can tolerate a somewhat slack chain, which is something of a culture shock, coming from a fixed gear.
cheers