WASHINGTON — President Donald Trump trails at least three Democratic hopefuls — Joe Biden, Bernie Sanders and Elizabeth Warren — in theoretical head-to-head matchups for the 2020 general election, a new poll showed. Biden, the former vice president, held the biggest lead among registered voters in the NBC News/Wall Street Journal poll released Sunday: 51 percent to 42 percent, well outside the survey’s margin of error, which was plus or minus 3.5 percentage points. Sen.