Substitution of Co and Ni for Fe in an Fe75P10C10B5 metallic glass effectively increases the stability of supercooled liquid and glass-forming ability. The bulk metallic glasses with large supercooled liquid region (ΔTx) exceeding 50 K and the critical sample diameter (dc) of over 1 mm were obtained in a wide composition range from 30 to 60 at.% Fe, 0 to 45 at.% Co and 0 to 40 at.% Ni in the (Fe, Co, Ni)75P10C10B5 alloy system by copper mold casting. An Fe40Co20Ni15P10C10B5 alloy shows maximum dc of 2.5 mm, largest ΔTx of 60 K, low glass transition temperature of 675 K, low viscosity on the order of 10⁸–10⁹ Pa s in the supercooled liquid state, high compressive yield strength of 2860 MPa with a distinct plastic strain, and good soft magnetic properties, i.e., rather high saturation magnetization of 1.13 T and low coercive force of 3.8 A/m. The nano-scale Fe40Co20Ni15P10C10B5 glassy patterns were precisely formed by imprinting, indicating its good thermoplastic formability.