We will present the rest-UV spectra of three radio galaxies at z~3 observed with VLT/FORS2 to measure the flux of several emission lines including relatively faint ones (such as OIII]λ1665, NIV]λ1486, and [NeIV]λ2424). In addition, we collect the UV-emission line fluxes of seven z~3 radio galaxies from the literature. We diagnose the physical and chemical properties of the ISM for each object through the comparison between the measured emission-line fluxes and detailed photoionization models with Cloudy. We confirm that the metallicity of NLRs in AGNs at z~3 is higher than the solar metallicity, without assuming the gas density and ionization parameter thanks to the newly detected faint emission lines. This result suggests that high-redshift radio galaxies have matured chemically at z~3.