A magnetic transition induced by shape anisotropy, geometry, and dipolar interactions has been found in a system of three single domain thin disks. The phase transition occurs only when the disks are in close proximity, and for a narrow range of aspect ratios. Near the transition, the system has an abrupt change from a closure-domain state with zero net magnetization to a magnetized state. The transition can be detected by changes in the hysteresis loops. Micromagnetic simulations with realistic parameters confirm the establishment of ferromagnetic ordering. © 2005 American Institute of Physics.