GREENVILLE, S.C. — Jeff Duncan, the popular and very conservative representative from South Carolina’s 3rd Congressional District, has never had three presidential candidates appear at his annual Faith and Freedom Barbecue. But tonight, at the Civic Center in Anderson, S.C., he’s welcoming Ben Carson, Ted Cruz, and Scott Walker — and fueling a lot of speculation about who has the best chance to win South Carolina’s first-in-the-South primary.