For your case, u can actually go to two idependent institutions namely, FIDREC & MAS.
FIDREC is like small claims tribunal(SCT) in the banking industry. It helps u to get your $$ back. But at a cost, $250 per claim if u do not succeed. $50 if u succeed.
MAS, I think u know what they do lah.... MAS cannot ask DBS to pay u back the money or waive the charge. A report to MAS in this case is to find if there are any breach or protocol, rules and regulations.
The most ambiguous point here is that the CSO might not actually have informed the bank of such fraudulent/unauthorized charge to your acc at the earilest point of time. Or if DBS was indeed informed by the CSO, they still approved the charge.
I'm not too sure if there is any breach of protocol/rules/regulations in this case as i do not come from the industry.
However, if indeed there was a breach of protocol/rules/regulations, then u might(at the bank's discretion) get the fraudulent charges waived.
I would suggest u lodge a report with MAS first(without incurring any cost), and wait.
If no breach of protocol/rules/regulations is found and DBS made firm their stand on not refunding u after all three transactions are cleared, then i suggest u go to FIDREC.
FIDREC's website: http://www.fidrec.com.sg/website/index.html
MAS's complaint website (scroll all the way to the bottom): http://www.moneysense.gov.sg/dispute...esolution.html
Hope it helps and good luck