  We investigate inclusive electron scattering reactions on two-neutron halo
nuclei in the quasielastic region. Expressions for the cross section and
structure functions are given assuming that the halo nucleus can be described
as a three-body system (${core}+n+n$). The method is applied to $^6$He. We
compute cross sections and structure functions, and investigate the kinematic
conditions for which the observables are determined either by $\alpha$-knockout
or by halo neutron-knockout. The optimal kinematical domain to disantangle the
momentum distributions of the various components of the three--body system ($q
\lesssim 200$ MeV/c and $\omega < q^2/2M_N + 20$ MeV) are explored.
