  In this work we report a detailed ESR spectroscopic study of the
single-crystalline samples of the van der Waals compounds
$M_{\text{2}}$P$_{\text{2}}$S$_{\text{6}}$ ($M_{\text{2}}$ = Mn$_{\text{2}}$,
MnNi, Ni$_{\text{2}}$), performed at an excitation frequency of 9.56 GHz, in a
broad range of temperatures above the magnetic order, and at different
orientations of the magnetic field with respect to the sample. Analyzing
temperature and angular dependences of the resonance field and of the linewidth
of the Mn$_2$P$_2$S$_6$ compound we have observed a significant change of the
spin dynamics from the dominance of the 3D-like fluctuations close to the
magnetic order to a relative increase of the 2D-like spin fluctuations at
higher temperatures. Such a behavior, which is opposite to the development of
the low-D signatures in the previously studied
Cr$_{\text{2}}$Ge$_{\text{2}}$Te$_{\text{6}}$ compound, can be explained by the
difference in the type of magnetic order in Mn$_2$P$_2$S$_6$ and
Cr$_{\text{2}}$Ge$_{\text{2}}$Te$_{\text{6}}$. On the other hand,
MnNiP$_2$S$_6$ compound exhibits angular dependences of the linewidth typical
for the system with 3D-like spin correlations in the whole measurement
temperature range, however the 2D-like correlations can be seen in the
temperature dependences of the resonance field and the linewidth.
Ni$_2$P$_2$S$_6$, in turn, does not show any 2D signatures. This suggests that
varying the Ni content in (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ one can control the
exchange interaction, possibly also in the third dimension.
