The trinuclear iron carbonyls Fe-3(CO)(n) (n = 12, 11, 10, 9) have been studied by density functional theory using the B3LYP and BP86 functionals. The experimentally known C-2v isomer of Fe-3(CO)(12), namely Fe-3(CO)(10)(mu-CO)(2), is found to be the global minimum below the unbridged D-3h isomer analogous to the known structures for Ru-3(CO)(12) and Os-3(CO)(12). The lowest-energy isomer found for Fe-3(CO)(11) is Fe-3(CO)(9)- (mu(3)-CO)(2) with iron-iron distances in the Fe-3 triangle, suggesting the one double bond (2.460 angstrom by B3LYP and 2.450 angstrom by BP86) and two single bonds (2.623 angstrom by B3LYP and 2.604 angstrom by BP86) required to give each Fe atom the favored 18-electron configuration. Two different higher-energy dibridged structures Fe-3(CO)(9)-(mu(2)-CO)(2) are also found for Fe-3(CO)(11). The lowest-energy isomer found for Fe-3(CO)(10) is Fe-3(CO)(9)-(mu(3)-CO) with equivalent iron-iron distances in the Fe-3 ring (2.47 angstrom by B3LYP or BP86). The lowest-energy isomer found for Fe-3(CO)(9) is Fe-3(CO)(6)(mu-CO)(3) with distances in the Fe-3 triangle possibly suggesting one single bond (2.618 angstrom by B3LYP and 2.601 angstrom by BP86), one weak double bond (2.491 angstrom by B3LYP and 2.473 angstrom by BP86), and one weak triple bond (2.368 angstrom by B3LYP and 2.343 angstrom by BP86). A higher-lying isomer of Fe-3(CO)(9), i.e., Fe-3(CO)(8)(mu-CO), at similar to 21 kcal/mol above the global minimum, has iron-iron distances strongly suggesting two single bonds (2.6 to 2.7 angstrom) and one quadruple bond (2.068 angstrom by B3LYP and 2.103 angstrom by BP86). Wiberg Bond Indices are also helpful in evaluating the iron-iron bond orders.