What an awful thing to happen. I'm so sorry, Alani.
This is a real puzzle. The window was high up and solid, as you say not an obvious weak spot as a target, and that corrugated plastic is pretty strong and it would be difficult to chew through or break by an animal jumping up at it. Whatever got in through the window must have got out again the same way, presumably. You say the hole was fox- sized - how big was that? I would have imagined it would be too small for a human to get through, and anyway a human would be more likely to force the door than break and crawl through a window. Was the door locked? Inside the coop, what did it look like, were the hens' bodies still in there, were there signs that they'd been injured by a predator, or could they just have been lifted off and passed out through the broken window?
Anyway, I can't imagine why a human would go to the trouble of stealing three domestic hens, can you? They're not very valuable, except to you, and hard to sell on in such small numbers. Sadly, there are some sickos out there who do this sort of thing, but as I said, it's hard to think that it was possible for them to actually get in to a relatively small and inaccessible hole. Any footprints outside, hair caught on the edges etc?