Here's my full panel. Plus a partial panel of a different type that had an ad on the left end of the front. The partial panel is blank backed.
The reason for 3 card panels is pretty simple, a 3 card panel fits nicely inside a folded brochure/letter/price list/order sheet inside a #10 envelope.
I'm thinking that maybe they were printed as their own print run. The panel with the ad would seem to bear this out. I don't have any of the 5 cards as regular cards, but I think it might be worth comparing the cards to the issued ones, as there is bound to be at least some difference.
Steve B