A three-orbital itinerant-electron model involving dxz, dyz and dxy Fe 3d orbitals is proposed for iron pnictides towards understanding the () ordered magnetism and magnetic excitations in these materials. It is shown that this model at half filling yields a gapped () magnetic state, and simultaneously reproduces several experimentally observed features such as the electronic structure, spin excitations, as well as the ferro orbital order between the dxz and dyz orbitals.