Oscar Insurance Corp. is starting to stem the bleeding after years of reporting large losses. The privately held health insurer, created to sell plans under the Affordable Care Act, lost $25.8 million across three states in the first three months of this year, compared with a loss of $48.5 million a year earlier, according to regulatory filings Tuesday. The company is beginning to get a handle on its medical costs, as the premiums it collected exceeded what it spent on health services.