Hi! I'm sorry to hear you're having this problem. As an avid Border Collie owner and former competitive shepherd, you're absolutely correct that it has a very high prey drive. That is one of those misconceptions the public has about what makes a dog want to herd other animals; it's part of prey drive. Usually, my Border Collies don't care too much for smaller animals, but they have been used for herding ducks and geese. So, they aren't completely indifferent to them either.
There are two things you'll have to do. First, be aware that you may actually loose a chicken while trying to gain control over the dog's will. You will need to start working with the dog on a long line and the chickens in a pen. The basic idea is praise the dog A LOT while he is behaving and not planning his afternoon snack. Give him simple tasks to do the entire time he is near the chicken pen, so you teach him to focus on his tasks, not his desires for dessert. There must be a flip side to this, as well, because you need to exert your dominance in the pack as Alpha. If he breaks and attempts to chase the girls, your correction needs to be rapid and strong enough to get his attention. Then, return him to doing other things and praising him for paying attention.
Keep your training sessions short, but frequent. Increase their length and difficulty over time and as the dog shows more trustworthiness. The dog is 3-yrs-old, already. So, this may be a long process. It should be achievable though.
My Border Collie is only interested in the chickens if they are roaming too far from their end of the property. He gives them a short bark and stomps his front paws. They quickly return to their B. Collie approved area and he goes on with his day. Honestly, he treats the cat with the same attitude. He doesn't like when she gets in the front of the chow line at feeding time. He reminds the cat of her place in the line and all is good in the world again.
Warning! DO NOT attempt to socialize the dog, as is often done with chickens, by leaving him chained where he can see the chickens, but is just out of reach of them. Though the birds may become acquainted that way, it has more of a tendency to drive the dog over mental cliff. The only thing that will be on the dog's mind after that will be KFC and mashed potatoes.
Good luck and have patience. Do not bend your rules you set out for the dog and praise him for colouring inside your lines. You are Alpha; he only gets chicken for dinner when you give it to him and his job is to ignore those girls.