Well first if you have anywhere to seperate the fish, I'd seperate the sick one out into a hospital tank. From what you said it could be any number of things...do the other two sharks pick on it?
What size of tank do you have them in? What temperature do you keep the tank at?
Has the sick one been eating?
If you dont have a choice of moving the fish, then I think I would raise the temperature of the tank and get a good multi-symptom medication going. Sorry i'm not going to be much help...hopefully someone else has more ideas for you...and if you answer the general questions and tell us as much about the fish and their tank and care as possible it will help out.
(welcome
)