You need to either pickpocket one of the people you gave a flask to...or kill one... There are only 2 flasks, you can not cure all 3 without doing one of the above.
you can pickpocket all three if you want, proviede you have that skill, <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />
or you could kill the guy who just wants to gamble his life away <img src="/ubbthreads/images/graemlins/suspicion.gif" alt="" /> like DATD DID! <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />