The monochromaticity improvement of green upconversion (UC) in Lu2O3:Yb3+/Ho3+ powders has been successfully realized by tridoping Eu3+. The integral area ratio of green emission to red emission of Ho3+ increases 4.3 times with increasing Eu3+ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb3+/Ho3+/Eu3+ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho3+ F-5(4)/S-5(2) level tothe Eu3+ D-5(0) level and ET from the Ho3+ I-5(6) level to the Eu3+ F-7(6) level. In addition, the population routes of the red-emitting Ho3+ F-5(5) level in the Yb3+/Ho3+ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb3+ F-2(5/2) level to the Ho3+ I-5(7) level and the cross relaxation process between two nearby Ho3+ ions in the F-5(4)/S-5(2) level and I-5(7) level, respectively, have been demonstrated to be the dominant approaches for populating the Ho3+ F-5(5) level. The multiphonon relaxation process originating from the Ho3+ F-5(4)/S-5(2) level is useless to populate the Ho3+ F-5(5) level. As the energy level gap between the Ho3+ I-5(7) level and Ho3+ I-5(8) level matches well with that between Eu3+ F-7(6) level and Eu3+ F-7(0) level, the energy of the Ho3+ I-5(7) level can be easily transferred to the Eu3+ F-7(6) level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho3+ F-5(4)/S-5(2) level to the Eu3+ D-5(0) level as well as the ET from the Ho3+ I-5(6) level to the Eu3+ F-7(6) level, the integral area ratio of green emission to red emission of Ho3+ has been improved significantly.