So sorry you've had this happen.
We had similar when we were very new to chicken-keeping. It was a local terrier, not the fox, but same devastation. Two dead, two wounded like yours, and the only one that had started laying had thankfully been hidden away in the nest box and survived unscathed.
We took the two wounded ones to the vet, who dispatched them, saying much what MS has just said about infected wounds and potential internal injuries.
Lacey became very tame, and befriended the sheep for a flock to follow, while we improved security and sourced new pullets.
Our new setup was dubbed Chicken Knox. We still used the timer door, but if we weren't about, they were shut in behind 6' netting, dug into the ground for 12", with electric (checked every day) - two strands at nose level and one at the top. When we were there, they free-ranged (and remained friendly with the sheep, led by Lacey!)
They were never any bother to lock up; Chicken Knox was large enough to still qualify as free range even if they'd been in there full-time, and they'd follow us anywhere if we had some bread
Here at BH's farm, the chooks have complete freedom and generally roost in the hay stacks. Cocky sleeps with an eye open and gives voice at any intruders. The downside of this arrangement is that they hide their eggs, so we have to buy our eggs off friends whose birds are penned!