It's the second national poll in as many weeks to have Carson in the top spot.
Bobby Jindal scored 6 percent support in the survey from Public Policy Polling.
Webb has been floating the possibility of running as an independent.
The Monmouth University poll differs wildly from others showing a tighter contest.
Carson is tops in favorability: 84 percent of Iowa Republicans view him positively.
Carson wins 28 percent in Bloomberg poll to Trump's 19 percent.
Clinton has an 11-point lead in new Quinnipiac survey of Iowa.
Donald Trump has a slight lead among GOP voters in the Sunshine State.
Poll is a welcome sign for Trump, whose rivals are catching up in other early-voting states.
Trump leads Carson by only 1 point in a Fox News national poll.