Saturday marks three years since Philadelphia mortgage insurer Radian Group Inc. announced a $5.5 billion deal to be bought by competitor MGIC Investment Corp. The proposed merger, driven by a slow housing market and announced amid the first tremors of the subprime-mortgage collapse, was a hint at the nation’s looming financial turmoil – though that was not obvious at the time. MGIC, of Milwaukee, walked away from the deal in September 2007, after the severity of the subprime-mortgage component of the crisis became evident. The future of Radian, which had bigger subprime losses than other mortgage insurers, was then cast into doubt by many in the industry. But Radian did not fail.