  We conduct a comprehensive study of three different magnetic semiconductors,
CrI$_3$, CrBr$_3$, and CrCl$_3$, by incorporating both few- and bi-layer
samples in van der Waals tunnel junctions. We find that the interlayer magnetic
ordering, exchange gap, magnetic anisotropy, as well as magnon excitations
evolve systematically with changing halogen atom. By fitting to a spin wave
theory that accounts for nearest neighbor exchange interactions, we are able to
further determine a simple spin Hamiltonian describing all three systems. These
results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin
classes in a single material family. Using magneto-optical measurements, we
additionally demonstrate that ferromagnetism can be stabilized down to
monolayer in more isotropic CrBr$_3$, with transition temperature still close
to that of the bulk.
