  For a model atom with the $p^3$ valence shell we construct consistent three-
and two-active electrons models enabling their direct comparison. Within these
models, we study the influence of the third active electron on the double
ionization yield in strong femtosecond laser fields. We reveal proportionality
between double ionization signals obtained with both models in the field
intensity region where non-sequential ionization dominates. We derive
analytically a correspondence rule connecting the double ionization yields
obtained within the three- and two-active electrons models.
