  The electronic structure and magnetic properties of the 2H-NbS$_2$ compound
intercalated by Cr, Mn and Fe, have been investigated by means of the
Korringa-Kohn-Rostoker (KKR) method. The calculations demonstrate easy plane
magneto-crystalline anisotropy (MCA) of Cr$_{1/3}$NbS$_2$ monotonously
decreasing towards the Curie temperature in line with the experimental results.
The modification of the electronic structure results in a change of the easy
axis from in-plane to out-of-plane. It is shown, that for Cr$_{1/3}$NbS$_2$ and
Mn$_{1/3}$NbS$_2$ the in-plane MCA and Dzyaloshinskii-Moriya interactions
results in a helimagnetic structure along the hexagonal $c$ axis, following the
experimental observations. The negative exchange interactions in the
Fe$_{1/3}$NbS$_2$ compound results in a non-collinear frustrated magnetic
structure if the MCA is not taken into account. It is shown, however, that a
strong MCA along the hexagonal $c$ axis leads to a magnetic ordering referred
to as an ordering of the third kind, which was observed experimentally.
