They're offered as a three unit set because that's how the Virginian ran them. As well, it's a heck of a lot easier for Scott and 3rd Rail to reach the minimum number of reservations to build selling 3 powered unit sets versus single powered units or one powered unit and two dummies.
They're not for everyone, but judging how the EA / E1 sets sold as well as the demand on this forum for the VGN jackshafts, I'm guessing these get made, and relatively quickly.
As an aside -- as non-aesthetically pleasing as the jackshafts were compared to steam or the diesels that followed them, they were a hugely important technological advancement at the time -- replacing three or more huge articulateds that were choking out their crews with smoke crawling up the 2% grade thru the tunnels west of Roanoke. Once that section of the railroad electrified, it never went back -- only after the merger did N & W dispose of the big electrics because they merged their routes and the Virginian's. I'm in for a set because they're a milestone in motive power as well as for roster completeness -- I have a model of literally all of the other Virginian big power except for these.