  Via the magnification bias, gravitational lensing by large-scale structures
causes angular cross-correlations between distant quasars and foreground
galaxies on angular scales of arc minutes and above. We investigate the
three-point cross-correlation between quasars and galaxy pairs measurable via
the second moment of the galaxy counts around quasars and show that it reaches
the level of a few per cent on angular scales near one arc minute. Combining
two- and three-point correlations, a skewness parameter can be defined which is
shown to be virtually independent on the shape and normalisation of the
dark-matter power spectrum. If the galaxy bias is linear and deterministic, the
skewness depends on the cosmic matter density parameter Omega_0 only;
otherwise, it can be used to probe the linearity and stochasticity of the bias.
We finally estimate the signal-to-noise ratio of a skewness determination and
find that around twenty thousand distant quasars e.g. from the Sloan Digital
Sky Survey should suffice for a direct measurement of Omega_0.
