Constant barking

Question
I have three dogs, a 7 year old female Husky, a 9 year old female Beagle mix, and a 5 year old male Chihuahua mix. They're all spayed/neutered. They bark constantly. I taught them all "quiet," and if just one of them is barking and the other two are outside, upstairs, etc., I can get the one barking to stop. But when they're all barking at once, they ignore me and I can't get them to stop. I've tried shaking a can with coins in it, clapping, making any noise to get their attention, but nothing works. If one of them starts barking, the other two bark too. My Chihuahua barks at every noise he hears and anything outside - people walking by, birds on the lawn, other dogs - so the other dogs will bark with him even if they don't know what they're barking at. What can I do?

Answer
Barking in a single dog is a signal (communication) and can be highly self rewarding (which is why some dogs bark a lot.)  In a pack, barking becomes a self perpetuating behavior; one dog barks, the others respond automatically.  This behavior must have been ongoing for quite some time since all your dogs are mature.  Your dogs appear not to be trained to "quiet" or they would be responding.  If you are perceived as psychologically highest in "rank" (I hate using that word but there's no other way to portray it), all dogs would (within a few seconds) stop upon your arrival to investigate and certainly upon your verbal communication to them.

The target of your intervention appears to be the Chihuahua.  This particular dog appears to be the alarmist.  The best way to control any behavior is to train it.  Teach the chi to bark on command (using positive reinforcement, see ClickerTraining.com or read Dr. Ian Dunbar's "How To Teach a New Dog Old Tricks"). Once the dog has obtained this skill (multiple trials, at least thirty, and then ten out of ten successful performances), stagger, and then remove, the reward for the bark and begin to teach 'quiet', rewarding that instead.  Whether or not the dog learns to respond depends upon his perception of you: kind, consistent and calm leadership makes any dog feel secure and under that umbrella your presence and subsequent intervention would stop him.  I can't see any of that from here, I have no idea how you live with these dogs.

Redirecting a dog from barking can also be achieved: train a behavior that is SO rewarding the dog will ALWAYS choose THAT: as in a "go to" behavior that is consistently rewarded with a substantial reward (like a fistful of cheese in tiny bites.)

You have apparently lived with this situation for quite some time and so have your dogs.  Successfully controlling over barking in a pack is a complex procedure dependent upon the dogs' socialization, training, trust in humans in their environment, and desire to follow/take redirection.  Asking someone online to cure a long standing problem is assigning that person an impossible task.