** Shares of CSL Ltd CSL.AX fall as much as 1% to A$127.85, its lowest since Aug 31, 2017
** Stock extends previous session's near-6% drop, on track for third straight day of losses
** U.S. defense department said on Tuesday that military personnel would no longer be mandated to receive the flu vaccine
** Biotech firm is heavily exposed to the U.S., its largest revenue source
** Firm's vaccines business, including influenza shots supplied through unit CSL Seqirus, is one of its most profitable divisions
** YTD, stock down 25.4%
(Reporting by Aamir Sheik Khalid in Bengaluru)
((Aamir.SheikKhalid@thomsonreuters.com))