  The problem of coupling multiple spin ensembles through cavity photons is
revisited by using PyBTM organic radicals and a high-$T_c$ superconducting
coplanar resonator. An exceptionally strong coupling is obtained and up to
three spin ensembles are simultaneously coupled. The ensembles are made
physically distinguishable by chemically varying the $g$ factor and by
exploiting the inhomogeneities of the applied magnetic field. The coherent
mixing of the spin and field modes is demonstrated by the observed multiple
anticrossing, along with the simulations performed within the input-output
formalism, and quantified by suitable entropic measures.
