  We determine the leptonic decay constants in three flavor unquenched lattice
QCD. We use O(a^2)-improved staggered light quarks and O(a)-improved charm
quarks in the Fermilab heavy quark formalism. Our preliminary results, based
upon an analysis at a single lattice spacing, are f_Ds = 263(+5-9)(+/-24) MeV
and f_D = 225(+11-13)(+/-21) MeV. In each case, the first reported error is
statistical while the is the combined systematic uncertainty.
