The troubled launch of Health Care.gov has raised plenty of questions about whether young, healthy people will enroll in coverage—and, if they don't, whether insurance companies will have to raise their premiums or give up on Obama- care's new insurance markets altogether. But the law's authors built in a safety net to help guard against that worst-case scenario. In essence, it's an insurance policy for insurance companies. The backstop is an approach known as the "three R's." And health care experts say that, taken together, the three prongs will help insurers not only grapple with the transition to the new...