I beg to disagree. The HST FAQ Book (collected from forum posts and mag interviews) specifically mentions 3 times a week, or 48-hour frequency in many places.
Although I do understand that it allows for e.g. Friday-Monday delay, which is 72 hours. So one may read that rule as "closest to 48 hours".
HST certainly does not impose 'must'. It imposes frequency. 3x is espoused in the ebooks because the ebooks were compiled from information and experiences of those who
did train at 3x per week.
I would advocate that the 36-48hr window is a guideline, albeit a highly accurate one. In addition, the focus ought to be removed from 'per week', that's merely a mental construction. For myself, I train every other day. MWFSuTuThSa-MWF... etc. It's proving highly effective, but I have also trained using 3x, 4x, 5-6x and 10-12x frequencies.
For the purposes of HST doctrine, 3x is a simple and easy mark to point to. It isn't however a set-in-stone requirement that, if not met, would invalidate the rest of the principles.
EDIT: I definitely advocate a minimum frequency of 3x per week if possible. My gut instinct is that people who don't gain on 3x + but do on 2x are simply not eating enough.
My 10-12x frequency used to look roughly like this:
AM:
Dips 2x
Chins 2x
Leg Press 2x
Shoulder Press 1x
Shrugs 1x
PM:
Incline 1x
Rows 1x
Leg Press 1x
Shoulder Press 1x
Shrugs 1x
And then invert the workouts each day so I'm doing 2x of Dips-Chins on M, then Incline-Rows on Tu etc etc.
I put on a reasonable amount of mass, but most notably when I adjusted diet a small amount I simply shed fat like nothing on Earth (being 21-23yrs old is a natural benefit to this sort of training as well, but I'm absolutely certain I could do it again at almost 29).
If one doesn't eat enough, they won't gain, and higher frequency raises your maintenance levels to accommodate for the extra exertion, let alone for gaining