My Friend I am very sorry for your loss. The kill description given so far suggest a canine. Canine includes a dog or fox. As there was no blood, and no one was missing or eaten on the spot it very much sounds like a dog. Recently a fox visited me and killed a number of chickens, but did haul one away. My guess is he killed with plans to come back. The fact that you have three injured and none hauled off/ missing suggests a small dog. A big dog simply would not have left one injured. A Fox would not leave on injured unless he thought it dead. Again I am sorry for you loss. It is important that you know that who ever killed will be back, either for the fun of killing again or to claim the kills left. Shore up your fences and keep your eyes open. Get the shot gun ready if you can.