Assuming the other two were "normal" 5x7 film holders, you got quite a good deal even if the three plate holders are useless.
I'm not sure how many flavors of Eastman plate holders there were, but if they have the same external form factor as the film holders, you can use them for sheet film simply by putting in an appropriate-thickness glass sheet as a spacer. The ones I have don't have a spring mechanism to position the plate, so the glass sits a little loosely and they might not be achieving absolutely precise registration of the film plane with the ground glass, but at the contact-print scale I've never seen any actual evidence of a problem.
Dry plates aren't very different in use from sheet film, but they have the benefit of rigidity (no flex problems) and the ability to be their own contact frames; on the other hand, they're heavy and they can get broken. Don't try taco-style development with them.
-NT