Hey there!
I would really take the advice of the others on seperating them. The infection will just continue to spread if they are kept together.... it will, realistically, go round and round in circles so even if one begins to clear up, its only a matter of time before the infection takes a hold again. As you have said, its a heartbreaking situation. Perhaps they are not biting on each other but you need to eliminate that possibility.
Your viv is a good size so there is no reason why you couldnt put a divider up in the middle and house one dragon on each side of it with one basking bulb on each side. For the other 2 dragons you could set up temporary enclosures using 2 seperate rubbermaid containers with a light suspended above each until you can get something else sorted for them. Remove any loose substrate and house them on paper towel for the time being which can be replaced daily. I would even consider removing any viv furniture and scrubbing it before replacing it with cardboard boxes until any infection present starts to clear up.
As Tracie said, use a betadine solution to
bathe them and the antibiotic ointment - from personal experience, it works wonders for cleaning up injuries and eliminaing infection.
I hope your beardies start getting better soon!
T