That's part of the curse of collecting Centaurs. Some of the issues are so rare that, despite cool covers, they're almost uncollectible because people are barely aware of their existence. These books are from April, May, and June of 1940, right in the sweet spot of early GA superhero books, and yet thanks to horrific distribution they are far less known than, say, Fantastic Comics #5-7 published in the same months (despite covers of comparable quality, in my mind).
You certainly put this in terms I can understand! (Still need #5 & #7 myself...maybe someday...)
There's a lot of overlap between Centaur and Fox collectors, so I figured comparing the publishers from the same month would help the scarcity perspective.
If I ever get the final 65 issues I need to finish my full Centaur collection, I'll likely move on to Fox.