WASHINGTON — The share of Americans without health insurance is dropping to the lowest levels since President Barack Obama took office, but sign-ups under his health care law lag among Hispanics.

With just three weeks left to enroll on the new insurance exchanges, the Gallup-Healthways Well-Being Index, finds that 15.9 percent of U.S. adults are uninsured thus far in 2014, down from 17.1 percent for the last three months — or calendar quarter — of 2013.

The drop of 1.2 percentage points in the uninsured rate translates to about 3 million people.

Gallup said the proportion of Americans who are uninsured is on track to drop to the lowest quarterly level it measured since 2008, before Obama took office.

“It’s probably a reasonable hypothesis that the Affordable Care Act is having something to do with this drop,” said Frank Newport, Gallup’s editor-in-chief. “We saw a continuation of the trend we saw last month; it didn’t bounce back up.”

The survey found that almost every major demographic group made progress getting health insurance, although Hispanics lagged.

With the highest uninsured rate of any racial or ethnic group, Latinos were expected to be major beneficiaries of the new health care law.

They are a relatively young population and many are on the lower rungs of the middle class, in jobs that don’t come with health insurance. They’ve also gone big for Obama in his two presidential campaigns.

Gallup found the biggest drop in the uninsured rate was among households making less than $36,000 a year — a decline of 2.8 percentage points.

The uninsured rate was down by 2.6 percentage points for blacks, 1 percentage point for whites, and eight-tenths of a percentage point for Latinos.