If you have rats, that may be what attracted the notice of the weasel and maybe even the fox. Weasels are one of nature's most effective rat killers, but they don't confine the carnage to just the rats, so while they may arrive for the rats, the chickens can be killed too.
For the rats, that is a long, multi-step process. Starts with elimination of food and harborage (places that shelter rats). Once you eliminate their food source, they they become receptive to eating poison bait blocks, which need to be served up from secure bait boxes. Bait boxes keep unwanted targets from finding the bait, and they also have pins that hold the bait blocks in place, so the rats can't drag them out into the open. Rats eat them there, then go home to die in their tunnels.
A coop secure enough to keep rats out will also keep the weasels out. Both can get through 1 inch holes, so chicken wire is no good. And that is wall to wall and treetop tall enclosures. Both can climb as well as dig.
Get rid of the rats, and you may get rid of your weasel. If not, the weasel box, if baited and set right, should do the trick.
And if rats and weasels can't get in, neither can a fox. If you are free ranging, then the fox has an open buffet going. He will happily keep coming back until he cleans you out. Rather than free ranging, you can setup a chicken yard, with a perimeter established by an electric fence. Done right, he will get zapped and decide no chicken is worth that and dine elsewhere.