A combination of erbium (Er), ytterbium (Yb), and neodymium (Nd), in fiber amplifiers, is examined. This combination extends the pumping wavelength spectrum by creating a new pumping path. Furthermore, the Nd addition enables pumping the amplifier from within the amplifier center, and thereby decreases the amplified spontaneous emission at the amplifier ends. A mathematical model is presented relating the excited population density, the signal power along the amplifier, the energy transfer among the ions, and the amplified spontaneous emissions. The study compares the amplifier characteristics of Er-Yb-Nd co-doped amplifier with a traditional Er-Yb amplifier for various combinations. The new configuration adds flexibility into the amplifier design. By properly selecting its parameters, one can increase the output signal power and decrease the amplified spontaneous emission, to comply with various requirements.