that really sucks... sounds like it may have been ich or some type of fungus. Either way that really sucks... sorry about your loss. You wouldnt happen to have any pics of this would you... or more descriptive so we can try to pin the cause or the problem
A pic would be good, My guesses are fungus, or a wound.
The 3 deaths is worrying, I don't think they were caused by the same thing, I think the pleco had a secondary infection causing the spot.
My guess (and that is all it is) is that you have internal bacteria or really bad water quality.have you got anything else in the tank?
if so I would do a waterchange.
If you can send a pic it will be put in the ID for desieses section once it has been diagnosed, so that people can look at it and see if they have the same problem - easy identification for desieses.