(In no particular order) Steven, Oliver, Jakub, David and Rod!
Thank you all for all those explanations.
I was mulling over this "by 3's" instructions for a long time without being able to wrap my head around it.
Oli's seminal comment "the unmilitary shape of the horse" did it for me. Often the Rules and Regulations state how things ought to be done but leave out the whypart. Blindingly obvious stuff to Napoleonic soldiers could be opaque to arm chair generals after 200 yeras.
Once again thanks all.