Dave, First and foremost I am sorry to hear about your woe's but I must congratulate you on your use of and reluctance to give up on proper quarantine procedures. Most in your situation would have blamed this process as at least partially to blame and moved on from there. Very happy your sticking with it.
As far as your potential fish problems, the first best step is identifying the problem and trying to find a possible solution from there. A "gunshot" approach is rarely going to be effective and you can often do more harm than good. This is going to require a good amount of detailed information on your part.
About the fish itself (species, behavior etc)
, your procedures/tank maintenance (Water changes, feedings, food types used etc)
, water quality and chemistry (ammonia, nitrite, nitrate, ph salinity temp etc)
, general set up (pumps, lights if any, decor etal...)
, tank mates if any,
symptoms the fish may be displaying (swimming, excessive gilling, spots, body texture, sores, redness and so on). Diagnosis is often a back and forth process of information so don't lose hope. The more information you provide, the faster the processs moves aalong so please do.
You will find that disease is not always the culprit in fish death so please do not limit your responses to just observations in that regard. As far as prophylactic treatments, lets leave that to the end of our discussion and focus strictly on the 3 remaining fish you have now and how they can be cared for. Once that's accomplished, we can go further in depth on other topics.
Cheers
Steve
