Hi @LeighPeaWelcome To BYC
I'm very sorry to hear about your troubles.
It does sound like you are dealing with a respiratory disease. If it's viral like Infectious Bronchitis or ILT, unfortunately, antibiotics usually won't help unless there is a secondary bacterial infection.
Tylan50 is used to treat secondary infections due to respiratory illness, but you need to make sure you are giving the correct dosage. Injectable Tylan50 canbe given orally or by injection - dosage is 1cc or ml per 5lbs of weight given 2times per day. Can you weigh your sick girls?
It won't hurt to see if the Tylan50 makes a difference once you adjust the dosage, it has a large dosage range and is fairly forgiving. The VetRx is a good idea - some people report it seems to help relieve some symptoms.
Do your best to keep them hydrated - if you have poultry vitamins add those to their water.
We also tend to want to close up chickens (like we would humans) when they are sick - but they do need fresh air and plenty of ventilation, but need to be kept out of drafts especially during roosting time. Since your birds have already been exposed, it's not necessary to keep them separated, except for the ease of giving supportive care/treatment.
I don't like to mention this, but if you do lose one, then having testing/necropsy performed so you can find out what you're dealing with would be a good idea. http://edis.ifas.ufl.edu/ps044
Please keep us posted.

