  We study the effects of spin degrees of freedom and wave function symmetries
on double ionization in three-electron systems. Each electron is assigned one
spatial degree of freedom. The resulting three-dimensional Schr\"odinger
equation is integrated numerically using grid-based Fourier transforms. We
reveal three-electron effects on the double ionization yield by comparing
signals for different ionization channels. We explain our findings by the
existence of fundamental differences between three-electronic and truly
two-electronic spin-resolved ionization schemes. We find, for instance, that
double ionization from a three-electron system is dominated by electrons that
have the opposite spin.
