When isolated spin clusters are weakly coupled to each other, zero-dimensional quantum and three-dimensional collective magnetism concomitantly occur. This is envisioned in the low-lying magnetic excitations, including transverse and longitudinal magnon modes along with gapped localized excitations. The newly discovered hexagonal perovskite Ba4NbMn3O12 constitutes Mn3O12 trimers made of three face-sharing MnO6 octahedra, with the trimers arranged in triangular planes. Ba4NbMn3O12 shows a ferromagnetic type order (possible ferrimagnetic order) at 42 K, yet with a small negative Curie-Weiss temperature of -4 K. In addition, the local spin alignment with the individual trimers occurs at much higher temperatures than the 3D long-range order. This energy hierarchy renders Ba4NbMn3O12 an attracting platform to explore a crossover of molecular magnetism to more collective behavior.