  (abridged) We present the X-ray analysis of a sample of 30 luminous quasars
at $z\simeq3.0-3.3$ with deep XMM-Newton observations, selected from the
SDSS-DR7 to be representative of the most luminous, intrinsically blue quasar
population. By construction, the sample boasts a unique degree of homogeneity
in terms of optical/UV properties. In the X-rays, only four sources are too
faint for a detailed spectral analysis. Neglecting a radio-loud object, the
other 25 quasars are, as a whole, the most X-ray luminous ever observed, with
rest-frame 2-10 keV luminosities of $0.5-7\times10^{45}$ erg/s. The continuum
photon index distribution, centred at $\Gamma\sim1.85$, is in excellent
agreement with those in place at lower redshift, luminosity and black-hole
mass, confirming the universal nature of the X-ray emission mechanism in
quasars. Even so, when compared against the well-known $L_{\rm X}-L_{\rm UV}$
correlation, our quasars unexpectedly split into two distinct subsets. About
2/3 of the sources are clustered around the relation with a minimal scatter of
0.1 dex, while the remaining 1/3 appear to be X-ray underluminous by factors of
$>3-10$. Such a large incidence ($\approx25\%$) of X-ray weakness has never
been reported in radio-quiet, non-BAL quasar samples. Several factors could
contribute to enhance the X-ray weakness fraction among our $z\simeq3$ blue
quasars. However, the X-ray weak objects also have, on average, flatter
spectra, with no clear evidence of absorption. Indeed, column densities in
excess of a few $\times10^{22}$ cm$^{-2}$ can be ruled out for most of the
sample. We suggest that, at least in some of our X-ray weak quasars, the corona
might experience a radiatively inefficient phase due to the presence of a
powerful accretion-disc wind, which substantially reduces the accretion rate
through the inner disc and so the availability of seed photons for Compton
up-scattering.
