The heat capacity of the isomorphic pseudo-one-dimensional magnetic systems CsMnCl3.2H2O, -RbMnCl3.2H2O, and CsMnBr3.2H2O has been analyzed using recently developed estimates for the lattice heat capacity of anisotropic media. The magnetic contribution in the paramagnetic region could be described very well by the S=52 linear-chain Heisenberg model. The intrachain interaction is found as Jk=-3.0 K for the former two compounds and Jk=-2.6 K for the latter compound. The interchain interactions are smaller by two orders of magnitude. The results, are compared with other experimental evidence. A study of the critical behavior of the magnetic entropy increase yields ==+0.100.01 for all three substances.