The one who is not sick has already been exposed so there's little chance of her not getting it. If you are able, you may want to get her away from the other two, just in case there's a chance she can get away with no catching it.

Kennel cough is a virus so the antibiotics don't help with it that much. They do help with the secondary bacterial issues so you DO still need them (particularly for that second dog). You will usually see the cough for 7-10 days so what you are seeing is not uncommon.

I would go ahead and boil some chicken and white rice and feed them this, including the broth.

You can give your vet a call and ask if he wants you to use an anti-tussin like robitussin. But I would not use it without the vet's permission.

Keep them well hydrated, get what food you can into them and you can use pediatric nasal saline in the nose to help clear mucus. You can also use plain pedialyte to keep them hydrated.

Then give your vet a call tomorrow to get antibiotics for that second dog.

Let me know if you still have questions.