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. The newly discovered hexagonal perovskite Ba4NbMn3O12 constitutes Mn3O12 trimers made of three face-sharing MnO6 octahedra, with the trimers arranged in triangular planes. Each Mn3O12 trimer is corner-shared with the non-magnetic NbO6 octahedra, thereby leading to weak intertrimer interactions through the Mn-O-O-Mn super-super-exchange paths. In addition, adBa4NbMn3O12 shows a ferromagnetic type order at 42K, yet with a small negative Curie-Weiss temperature of -4K. This energy hierarchy renders Ba4NbMn3O12 an attracting platform to explore a crossover of molecular magnetism to more collective behavior.