  We present high S/N UV spectra for eight quasars at z $\sim$ 3 obtained with
VLT/FORS. The spectra enable us to analyze in detail the strong and weak
emission features in the rest-frame range 1300-2000 A of each source
(Ciii]$\lambda$1909, Siiii]$\lambda$1892, Aliii$\lambda$1860,
Siii$\lambda$1814, Civ$\lambda$1549 and blended
Siiv$\lambda$1397+Oiv]$\lambda$1402). Flux ratios
Aliii$\lambda$1860/Siiii]$\lambda$1892, Civ$\lambda$1549/Aliii$\lambda$1860,
Siiv$\lambda$1397+Oiv]$\lambda$1402/Siiii]$\lambda$1892 and
Siiv$\lambda$1397+Oiv]$\lambda$1402/Civ$\lambda$1549 strongly constrain
ionizing photon flux and metallicity through the use of diagnostic maps built
from CLOUDY simulations. The radius of the broad line region is then derived
from the ionizing photon flux applying the definition of the ionization
parameter. The rBLR estimate and the width of a virial component isolated in
prominent UV lines yields an estimate of black hole mass. We compare our
results with previous estimates obtained from the rBLR - luminosity correlation
customarily employed to estimate black hole masses of high redshift quasars.
