Three of Ambac Financial Group Inc’s (AMBC.O) top ten shareholders are calling on its new CEO Nader Tavakoli to step down, according to people familiar with the matter. One of the reasons they give is that he is slow to settle $4 billion in insurance claims in which at least two of them have an interest, the sources said. The policy claims relate to complex financial instruments, including secured mortgage backed securities, that were insured by Ambac and were at the center of the 2008 financial crisis.