I think your situation kind of lies between the two main dog 'arguments.' I agree that a dog can be trained....to an extent. But, your Collie has already gone after the chickens multiple times which is what creates the issue, I think. He has already acquired a 'taste' for whatever is attracting him to the birds, be it the chase, the promise of a snack, or just aggressive behaviour toward them for an unknown reason. My personal opinion is that it is absolutely not ok for dogs to chase chickens. How can we know that the dog is herding them rather than just chasing to chase? It's too hard to differentiate in these situations because unfortunately we can't speak to dogs. It's also not something that should be guessed at when another pet's life could be at stake. My dogs will not be allowed to move sharply toward the chickens at all. If training does not seem to be going well, they won't even get the chance to be off a leash near the chickens and will just have to stay in side while the girls free range.
I hate to say it, but I believe your Collie has probably formed a habit that will be hard to be broken. You can try to train him, but it's doubtful it would work. I would definitely not leave the chickens and dogs together unsupervised at all, even if you do manage to train. For now I think your best bet is not letting the dogs out while the chickens are out and vice versa.
For training, a strong leash, a second person, and an open area is what I would recommend. You don't want to stress your chicken(s) anymore than they must already be because of the dog. Keep the dog close to you, use a sharp voice, and let the chickens run away if they need to. It is possible, but I wouldn't get too hopeful as it's kind of late to start training. I feel training is a long drawn out process that should start from the day you bring the chicks home. You can't force it, so if doesn't seem to be going well, figure out an alternative. Having a dead chicken at the fault of another pet does not seem like a good place to be in. Dogs will be dogs. It's their nature. Without completely breaking them, it's impossible to get rid of those instincts.
I think the best thing you could wish for is for the dogs to realise it's not ok to chase or attack while you're around and just never leave them unsupervised together, even for a few minutes. Even a trained dog sometimes breaks the rules because they are smart. When you train an animal, the biggest thing you've trained them to do is not misbehave in your presence.