Location of the 30 quasars at z ≃ 3.0 − 3.3 analysed in this work (blue stars) in the luminosity–redshift plane of the SDSS-DR7 catalogue (Shen et al. 2011), where radio-loud and BAL quasars have been removed (91 484 objects, grey dots). Also shown for comparison, with the same filters applied, are the remaining 25 sources from the WISSH sample (6 more are in common) with available X-ray data (in orange, from Martocchia et al. 2017; the objects for which the number of counts was enough to perform a spectral analysis are flagged with red circles).