WASHINGTON — The nation’s health care law is approaching limits as its third sign-up season gets underway. Enrollment on the federal and state exchanges began Sunday. President Barack Obama’s health overhaul already has reduced the uninsured rate to a historic low of about 9 percent, but the gains will be harder in 2016. Costs are going up on the private, taxpayer-subsidized coverage sold through HealthCare.gov and state insurance exchanges.