You're going to need to spend a lot of time with each dog alone each day, in pairs, and in a trio on check cords (long leash... not the retractable kind necessarily) or collars as others have mentioned... Consider putting your chickens in some sort of secure cage or tractor in the middle of your yard to keep them safe while you're training. The INSTANT the dog looks at the chicken for ANY reason, correct hard with a "LEAVE IT" before any look has a chance to flip into anything else (heartbeat)- be consistent, extremely firm, and unequivocally THE BOSS... You may not have to do as much of this with the older dog - but NEVER forget that they are at their core, bird hunting dogs, which may override all of this in a snap if there is a lapse in your training diligence. Terriers have strong tendency to obsess, so nip it sharply before it has a chance to move into that... The next step is to make the dog lay completely on it's side or roll it to it's back (belly up) wait until the dog is completely submissive and with protections in place allow the (a) chickens to come to the dog - do NOT let the dog even look at the chicken, or allow it to change the submissive behavior even in distant mojo...
Eventually the chickens will become a 'non event', although with three dogs you certainly have the danger of the pack thing, so there may never be a time when you will be able to leave them alone. Of course each dog is different, so it's really up to you to read them and work properly with each dog's temperament.
That said, this is certainly not the time for touchy feelee clicker training... it's time to lay down the law.
This worked like a champ for my two JRTs. One of them acts as if the chickens don't remotely exist, the other knows the law and has made it her job to be the chief chicken-keeper. She and our Roo had a venomous mutual dislike - I let them sort it out, and they found it best to call it a draw.
Good Luck!
