LONDON/FRANKFURT (Reuters) -Deutsche Bank shares fell for a third day on Friday, after a sharp jump in the cost of insuring against the risk of default late the day before fuelled concerns about the overall stability of Europe's banks. Deutsche shares, which have lost a fifth of their value so far this month, were last down 5.5% at 8.843 euros ($9.57), not far off Monday's five-month low. They closed 3.2% lower on Thursday, while the bank's credit default swaps (CDS) - a form of insurance for bondholders - shot up to 173 basis points (bps) from 142 bps the day before, according to data from S&P Market Intelligence on Thursday.