Recently, InGaP/GaAs/Ge 3-junction solar cells are widely used for space because of their higher conversion efficiency and better radiation-resistance compared to GaAs and Si solar cells. In this study, effects of base carrier concentration in GaAs and InGaP sub-cells upon their radiation resistance are analyzed by using radiative recombination lifetime and damage constant K for minority-carrier lifetime of GaAs and InGaP. In addition, analytical results are also compared with the experimental results of InGaP solar cells irradiated with 1-MeV electrons, 30-keV and 200-keV protons. In low irradiation fluence, n-on-p structure cells are found to be more radiation resistant than p-on-n structure cells. Better radiation-resistance of sub-cells can be realized by optimal design based on fundamental approach for radiative and non-radiative recombination properties of InGaP and radiation-resistance of InGaP/GaAs/Ge 3-junction cells will also be improved by optimal design of sub cells.