The anisotropic boron atom electron paramagnetic resonance spectra measured in rare gas matrices (Ar, Kr, Xe) are interpreted with the aid of highly correlated ab initio calculations including spin-orbit coupling and diatomics-in-molecules (DIM) molecular dynamics simulations. The heavy-element and crystal field effects are inspected as they contribute to the electron g-shift. The DIM-simulated p-orbital splittings and lattice perturbed hyperfine coupling values provide a good starting point for spectral fitting and show the correctness of the guidelines given by purely synthetic generation of the spectra. The present combination of experiment and theory resulted in improved accuracy of the parameters measured in Ar matrix, new values are extracted for Kr matrix, and tentative assignment is also provided for the Xe matrix case.