[Arran]

3way power is a reference to the fact that you can power it from 117v ac 117v dc or batteries. Sales buzz word.

  I think that it was pretty much a sales pitch, at one time it was relevant as there were DC mains power grids in places like down town N.Y.C, it's the same with most transformerless series string sets, commonly referred to as AC/DC sets in their marketing. I would guess that even back in the 1930s having to deal with a DC power grid was only of concern to probably 5% of their customers, but it sounds better in marketing then promoting a set as being cut rate thanks to the omission of a power transformer. Believe it or not I have seen tube, battery portables with a power transformer, but they were either British or European made, not North American made.
Regards
Arran