Well hopefully if you ask neighbors, some of them survived. Its often the case that if they had a means to flee -- while the dog is killing one the others are heading for "cover". Check with neighbors and let them know you are missing 3 as feathers may only mean the dog got a mouth full of feathers. NOT the bird. Finding body parts is the only way you can be sure they are dead.
My aunt had an interesting way to deter dogs on the farm from killing her free range hens. She trained each dog using a bundle of wing feathers that she would hold in one hand, while having the dog on a leash. She would take the dog into the pullet coop and walk the dog. When it lunged at the birds, it got wacked with the feathers. Over and over this would go on and soon, the FEATHERs were seen by the dog as "not cool". Every dog that was to come onto the farm went through their indoctrination and it worked every time. No harm (physically) to the dogs but mentally they got the picture.
After that the dogs & the chickens could have the run of the farm yard with no problems.
Anisah (in South Dakota)