No, a fox can jump a 10 ft. fence. I once saw a video of a chicken house with a 10 ft. fence, barbed wire at the top and a strand of electrified wire out from the bottom. After several attempts, the fox was able to jump to the top of the fence, avoiding the electric and get into the run.
But if the fox jumps the 3' fence, the chickens can fly to the roof or up in a tree or just outrun the fox long enough to reach safety.
Sure they can go over a 10' fence. BUT, they will not go over the electrified poultry net. A fox, like any other kind of animal, always checks things out with their nose. And that is a very bad thing with the poultry net and it's charge of over 7000Vs. Once they get a taste of that, they don't even approach the fence anymore. My fence has been up close to 4yrs now. I've got fox, coon, dogs, just about everything short of a bear running through here, and I have not lost a bird. The birds are out everyday, with no problem. I've seen the fox, just standing out there looking at the chickens. But he won't come any closer than 20' to that fence, (And if I can get to my rifle in time, that will be the last thing he looks at)