I don't know about where you are, but some of the local pet shops here will tell you that a 5 gallon tank can house five goldfish and that Pacus are fine in a 10 gallon. I would humbly suggest that you do a little research on your own. Certain loaches are fine with goldies. Other algae eater types (which are also sold under the generic term "loach" in some places) are NOT so fine with goldies and can get territorial (and large) as they get older.
If it is a water issue and not an aggression issue, then all three of the goldfish should be showing some signs of fin damage. You have a lot of fish in a very small tank. It would not be too surprising if you have territory issues. If you have two mature male goldfish in a tank that is too small, they can become aggressive with each other over a female, and goldfish are fairly passive.
If it is available to you and affordable, a water testing kit of your own would be an excellent investment. Water parameters at that stocking level can CERTAINLY change day to day.
I wish you the best of luck with your fish. In any event, if the fish shows signs of infection at the tail, I would quarantine it and try treating with fresh water and salt, probably at about 1 teaspoon/gallon. I would only try other treatments if the symptoms worsen quickly. There is a good chance that a nipped or burnt tail will grow back.