I've got a pleco with my goldies, no problems
Here, from the looks of it, this fishes' slime coat is incredibly thick. Probably protecting it from parasites or something bad in the water. How is the second fish holding up?
Now I'd be changing the water regularly, adding tons of beneficial bacteria to the water. I would recommend "fluid filter media" by EasyLife since it helps the fish heal smaller wounds at the same time. I would also treat the water with normal, medical aquarium salt according to the package since this will help against almost anything
Do a 75% water change before adding the salt and leave it alone for a week, just keep an eye on the fish.
Feed the fish some anti-parasitic food as well, and DO NOT buy any more fish. Your tank is full with these three fish now.