A buzzard can be a hawk.  A buzzard can be a vulture.  Buzzard is one of those words that can mean different things to different people.  Once again we are separated by a common language.  My Kansas wife was absolutely convinced that a buzzard was a hawk.  She got mad at me when I pulled out the dictionary and showed her buzzard could be either hawk or vulture. 
Just because you did not see blood or feathers does not mean it wasn't a fox or a dog.  If they took three at a time I would not suspect hawks, buzzards, ernes, eagles, owls or other birds of prey.  The critters that often leave no traces and take multiples are often canines or humans.  Yes, canines like fox or dog often do leave clues like feathers, but not always, especially if Mama Fox is feeding a bunch of young back at the den.  Or a dog may carry them off and line them up, just for fun.  I once has two dogs take 8 chickens.  I found one body, two other piles of feathers, but five disappeared with no trace.   I'll leave humans to your imagination.
If you don't keep them in a pen with a top, they are vulnerable to birds of prey.  The best defense is to give them as much cover as you can so they have a place to run to if a bird of prey is spotted.  That can be bushes, low hanging trees, the coop, or outbuildings.  A raised deck can help.  Someone took an old satellite dish and propped it up so they could get under it. 
I have red-tailed hawks all over the place her, big ones.  I've seen a bald eagle a few times.  They just haven't been a problem, but some people do lose a lot of chickens to birds of prey.